≡× MENU

• Repair
• Repair
• Interior Design
• About everything
• About plumbing

The update data modification operator allows. SQL(DML) help: UPDATE and DELETE statements. VBA Data Types

Operator UPDATE changes existing data in a table. The command has the following syntax

UPDATE<имя таблицы>
SET (column name = (expression to calculate the value of the column
| NULL
| DEFAULT),...)
[ (WHERE<предикат>}];

With a single statement, values ​​can be set for any number of columns. However, in the same statement UPDATE you can make changes to each column of the specified table only once. In the absence of an offer WHERE all rows of the table will be updated.

If the column allows a null value, then it can be specified explicitly. Alternatively, you can replace the existing value with the default value ( DEFAULT) for the given column.

Naturally, column data types hd And ram must be compatible. An expression can be used to cast types CAST.

If you want to change the data depending on the contents of a certain column, you can use the expression CASE. If, say, you need to put 20 GB hard drives on PC notebooks with less than 128 MB of memory and 40 GB hard drives on other PC notebooks, then you can write the following query:

A few words need to be said about auto-incrementing columns. If column code in the table laptop defined as IDENTITY(1,1), then the next operator

Of course, another line with the value code=5 should not be in the table.

In Transact-SQL statement UPDATE extends the standard by using an optional clause FROM. This clause specifies a table that provides criteria for an update operation. Additional flexibility here is provided by the use of table join operations.

Example . Let it be required to indicate "No PC" (no PC) in the column type for those PC models from the table product PC. The table join solution can be written as follows:

DELETE statement

Operator DELETE deletes rows from temporary or permanent base tables, views, or cursors, and in the last two cases, the action of the operator extends to those base tables from which data was retrieved into these views or cursors. The delete operator has a simple syntax:

DELETE FROM<имя таблицы > ;

If the offer WHERE missing, all rows are removed from the table or view (the view must be updatable). You can also perform this operation (deleting all rows from a table) faster in Transact-SQL using the command

However, there are a number of differences in the implementation of the command TRUNCATE TABLE compared to using the operator DELETE things to keep in mind:

1. Deletion of individual table rows is not logged. The log records only the freeing of pages that were occupied by table data.
2. Triggers do not work, in particular, the trigger for deletion.
3. The command is not applicable if this table is referenced by a foreign key, and even if the foreign key has the cascade delete option.
4. Counter value ( IDENTITY) is reset to the initial value.

Example . Need to be removed from table laptop all PC notebooks with a screen size of less than 12 inches.

TRUNCATE TABLE Laptop

Transact-SQL Extends Statement Syntax DELETE, introducing an additional sentence FROM

FROM<источник табличного типа>

With help table type source you can specify the data to be removed from the table in the first sentence FROM.

With this clause, you can perform table joins, which logically replaces the use of subqueries in the clause WHERE to identify rows to be deleted.

Let us explain what has been said with an example. Suppose you want to remove those PC models from the table product, for which there are no corresponding rows in the table PC.

Using the standard syntax, this task can be solved with the following query:

Here an outer join is used, resulting in a column pc.model for PC models not listed in the table PC, will contain a NULL value, which is used to identify the rows to be deleted.

The SQL Server platform supports most of the major components of the ANSI UPDATE statement, but the ONLY and ARRAY keywords are not supported, and the ability to update arrays is not supported. In SQL Server, the functionality of the UPDATE statement has been enhanced by adding table hints with the WITH clause, adding query hints with the OPTION clause, and more robust variable handling. The syntax is the following.

UPDATE (table_name | view_name | row-set) )] SET (column-name=(DEFAULT | NULL | scalar_expression) |

stirrup name=scalar_expression| variable_name=column_name=scalar_expression) [, …] ]

WHERE (conditions | CURRENT OF cursor_name) )]

The syntax elements of an UPDATE statement in SQL Server are as follows.

WITH hint

Allows you to use table hints, overriding the default behavior of the query optimizer. Because the query optimizer is very good at choosing processing plans, use hints only if you have a very good understanding of the tables, indexes, and data affected by the operation. If there is no such understanding, the use of hints can lead not to an increase, but to a decrease in performance.

variable_name

SQL Server variables must be declared prior to using an UPDATE statement, in the form DECLARE @variable. The SET construct @variable=column!=expression! sets the variable to the final value of the updated column, and SET @variable-column!, column!=expression sets the variable to the value in the column until the UPDATE statement is executed.

Provides the ability to create a highly selective join-based criteria for specifying which rows to update. The FROM clause is not needed if only one table, the target table, is used when specifying rows. Functions for working with rowset functions in SQL Server are described in the "SELECT statement" section.

AS alias

Allows you to assign an easy-to-use alias to a table, view, nested table subquery, or rowset function.

It is possible to use standard ANSI syntax for table joins in conjunction with the FROM clause.

A slight variation on the ANSI WHERE CURRENT OF clause. The WHERE CURRENT OF clause of a cursor name used in combination with a cursor causes the SQL Server platform to update only the one record on which the cursor is currently located. The cursor is assumed to be local, but you can also specify a global cursor using the GLOBAL keyword.

OPTION prompt

Allows you to use query hints by changing the default behavior of the query optimizer. As with the WITH clause, use hints only if you have a very good understanding of the tables, indexes, and data affected by the operation. If there is no such understanding, the use of hints can lead not to an increase, but to a decrease in performance.

The main extension that Microsoft SQL Server introduces to the ANSI standard UPDATE statement is the FROM clause. The FROM clause allows you to use the JOIN clause, which greatly simplifies updating the rows of the target table by associating the rows specified in the FROM clause with the rows updated by the table_name UPDATE component. The following example shows updating the result of a table join using ANSI style and a rather unwieldy subquery, and then updating it using the SQL Server FROM clause. Both requests perform the same action, but in completely different ways.

Performing such an update using Transact-SQL style is a matter of joining two tables, authors and titleauthor, to the titles table. To perform the same operation using ANSI code, you first need to look up the au_id value in the authors table and pass it to the titleauthor table, and then look up the title_id value and pass it to the main UPDATE statement.

The following example updates the state column for the first ten records in the authors table.

UPDATE authors SET state="ZZ" FROM (SELECT TOP 10 * FROM authors ORDER BY au_lname) AS t1 WHERE authors.au_id=t1.au_.id

In this example, it is important to note that it is generally quite difficult to update the first n records with an UPDATE statement unless there is some explicit sequence of rows that can be determined with a WHERE clause. However, the nested table subquery in the FROM clause, which uses the TOP keyword to get the first 10 records, saves you from wasting extra programming that would otherwise be necessary.

Changing the value of fields (command UPDATE). Komatsdoy UPDATE you can change some or all of the values ​​in a row. The command specifies the table name and changes. Suppose, by order of the rector, all students who receive a scholarship are given a new (same for all) scholarship amount of 50,000 rubles. For this, the command is given:

UPDATE Spisok

SET stip = 50,000;

In a team UPDATE in a sentence WHERE you can specify to update only certain rows.

UPDATE List

SET stip = 50,000

WHERE curs=I;

In the SET clause of the command UPDATE You can specify any number of values ​​for columns separated by commas.

One team UPDATE You cannot update multiple tables.

In a team UPDATE in a sentence SET you can use scalar expressions to change the value of a field. Suppose all students receive a 25% increase in scholarships; changes need to be made:

UPDATE Spisok

SET stip = (stip *25)/100;

So the team UPDATE is designed to replace values ​​in rows (records) of a table.

SQL. DELETE Command Syntax

Deleting rows from a table (command DELETE). team DELETE not individual row field values ​​are deleted, but entire rows. After executing the command DELETE for the whole table, it becomes empty, for example:

DELETE FROM list;

To delete specific rows, you can use the predicate

DELETE FROM Spisok

WHERE tip = 0;

or specify a primary key value to delete a single record

DELETE FROM Spisok

WHERE fio = "Petkin";

In teams INSERT, DELETE, UPDATE subqueries can be used.

SQL SELECT Command Syntax

The SQL language is a structured query language. A query in this environment is a command with which the user generates a task for the DBMS. After executing the command, the DBMS must provide the information specified in the request to the user.
all requests in SQL formulated with a single command SELECT, after which the search for certain (necessary) information in the table begins in the database.

Example. Based on the Spisok table, get a table with all records of the following form:

The command is given:

SELECT fio, stick FROM list;

The result of the command execution is shown in the figure.

fio	stip
Rybkina
Utkin
Pshenichko
Sobachkina
Testicle

Select- a keyword that "tells" the DBMS that the command is a query;

fio, stick- list of names of fields (columns), according to which information should be selected and the new table should be normalized;

FROM Spisok; FROM- keyword, must be in every request; Spisik - the name of the table that is the data source for the query;

The semicolon (;) symbol indicates the end of the command and the readiness to execute it.

After keyword SELECT followed by a space. Further, the names of the fields (columns) of the selection are listed separated by commas.

To display all columns of a database table, the list of fields can be omitted by replacing it with an asterisk (*). Columns are displayed in accordance with the structure of the source table

SELECT * FROM list;

The SELECT command can display columns in any order other than the ordered order of the source table structure. This sequence is specified by the list of column names in the SELECT command. An example of reordered columns in an output table

SELECT Kurs, gruppa, fio FROM list;

SQL. Selection by criteria (WHERE)

Using the app WHERE in a team SELECT the condition for selecting records from the table is set; the predicate can take the values ​​"true" or "false".

Example.

Team SELECT to select students from the ET-41 group from the Spisok table:

SELECT gruppa, fio FROM Spisok WHERE gruppa='ET-41';

The program will look through all records of the Spisok table, checking each of them for the truth of the predicate gruppa='ET-41'. As a result of execution we will receive.

gruppa	fio
ET-41	Rybkina
ET-41	Utkin
ET-41	Pshenichko
ET-41	Sobachkina
ET-41	Testicle

Using SQL with other programming languages

Language SQL used to write database access programs as a sublanguage of other procedural programming languages ​​(Pascal, PL/1, Fortran, COBOL, C, Ada, etc.). Language SQL not procedural, but declarative, local. It can be embedded in programs written in procedural languages. in tongue SQL there are no basic IF condition check statements, FOR, DO and WHILE statements, etc. Language SQL designed exclusively for database management.

The combined use of these languages ​​allows you to program complex procedural programs, for example, programming them in Pascal, and through SQL access their database. All built-in commands SQL are included in the main text of the program in a high-level language, begin with the phrase EXEX SQL and end with the termination sign of the corresponding language (in Pascal;).

Program with built-in SQL before its own compilation, it undergoes precompilation, where the commands SQL are converted (translated) by the precompiler into the form of a high-level language; after that, the entire main program is compiled.

Teams SQL included in another program are treated by the main program as SQL-procedures. During execution, the main program communicates with the database, just like a user in the environment SQL.

Visual Basic Application. Basic concepts.

In programming, the basic concepts are "variable" and "value of a variable". A variable is a value (object) whose value changes during program execution. A variable in a computer is represented by its assigned name (identifier), and its value is determined by an assignment operator.

For each variable identifier, a place is allocated in memory where its value is stored. Let's remember and understand, at first glance, an elementary concept: any program uses variables and their values. For each new variable name encountered in the program, VBA automatically determines it (allocates memory space for it). This space remains empty until a value is assigned to the variable. Variables are compared to containers for storing data of any type.

Data assigns an object in numbers, letters, and other ways. Arithmetic operations are performed on numbers, logical operations are performed on letters. This already indicates different types of data, for each of which their own methods are accepted inside the machine representation.

VBA Data Types

Data types	Prefix	Description symbol	Size (byte)	Meaning
Integer	int	%		Short Integer (-32788 to +32767)
Long	lng	&		Long Integer (-2147483648 to +2147483647)
Single	Sng	!		Single precision floating point number: -3.4e38 to +3.4e38
Double	Dbl	#		Double precision floating point number: -1.79e308 to +1.79e308
bytes	byt	No		Byte: 0 to 255
Currency	Cur	@		Fixed point number: -922337203685477.5808 To +922337203685477.5807
String	str	$	10 + 2 per character	String: 0 to 65535 characters
Boolean	Bin	No		boolean number: true and false
Date	Dat	No		date and time
Variant	Var	No	<16	Any data
Object	obj	No		Any object reference

Variables and constants in VBA

Variables in the program are declared, that is, the type of the variable and the scope are determined. Variables can be declared at two levels - the procedure level and the module level.

The declaration at the module level is made by the Public and private operators: at the module and procedure levels - by the Dim operator: only at the procedure level - by the static operator.

Initialization of declared variables occurs at compile time: a numeric variable is assigned the value zero (0); a string variable is an empty string (zero length); a variable of type Variant is an Emtry value (no value).

Constants: have a name but do not change their value during program execution. To define constants in VB, the Const statement is used in this format:

Const (const name =<константное выражение>}…

Public - the declared constant is available to any procedure

Private - for procedures within a module

AS<тип данных>- constant data type declaration

Last update: 07/13/2017

The UPDATE command is used to update existing rows in a table. It has the following formal syntax:

UPDATE table_name SET column1 = value1, column2 = value2, ... columnN = valueN

For example, let's increase the price of all products by 5000:

UPDATE Products SET Price = Price + 5000

Use the criteria, and change the manufacturer name from "Samsung" to "Samsung Inc.":

UPDATE Products SET Manufacturer = "Samsung Inc." WHERE Manufacturer = "Samsung"

A more complex query - replace the Manufacturer field with the value "Apple" with "Apple Inc." in the first 2 lines:

UPDATE Products SET Manufacturer = "Apple Inc." FROM (SELECT TOP 2 FROM Products WHERE Manufacturer="Apple") AS Selected WHERE Products.Id = Selected.Id

A subquery after the FROM keyword fetches the first two rows where Manufacturer="Apple". The alias Selected will be defined for this selection. The alias is specified after the AS statement.

Next comes the update condition Products.Id = Selected.Id . That is, in fact, we are dealing with two tables - Products and Selected (which is derived from Products). Selected contains the first two lines where Manufacturer="Apple". In Products - generally all lines. And the update is performed only for those rows that are in the Selected selection. That is, if there are dozens of products with Apple manufacturer in the Products table, then the update will affect only the first two of them.

UPDATE Command- makes changes to an already existing record or to multiple records in a table SQL. Changes existing values ​​in a table or in the view's main table.

UPDATE Command Command Syntax

UPDATE Command Syntax

UPDATE command. Basic keywords and parameters of the UPDATE command

• schema- authority identifier, usually the same as some username
• tableview- table name SQL in which the data changes; if a view is defined, the data is changed in the main table SQL representation
• subquery_1 - subquery, which the server treats in the same way as the view
• Witholumn - table column SQL or views SQL, whose value is changing; if table column from clause SET omitted, column value remains unchanged
• expr - ; this expression can contain main variables and optional indicator variables
• subquery_2 - the new value assigned to the corresponding column
• subquery_3 - the new value assigned to the corresponding column

WHERE- defines the range of rows to be modified by those for which the specified condition is TRUE; if this phrase is omitted, all rows in the table or view are updated.
When issuing an approval UPDATE any UPDATE trigger defined on the table.
Subqueries. If the offer SET contains subquery, it returns exactly one row for each row being modified. Each value in the result of the subquery is assigned to the corresponding columns of the list in parentheses. If the subquery does not return any rows, the column is assigned NULL. Subqueries can select data from the modified table. Offer SET can combine expressions and subqueries.

UPDATE Command Example 1
Changing the rating for all buyers to a value equal to 200:

UPDATE Customers SET rating = 200;

UPDATE Command Example 2
Replacing the value of a column in all rows of a table is generally rarely used. Therefore, in the team UPDATE like in a team DELETE, you can use the predicate. To perform the specified substitution of the values ​​of the rating column, for all buyers who are served by the seller Giovanni (snum = 1003), enter:

UPDATE Customers SET rating = 200 WHERE snum = 1001;

SQL UPDATE Command Example 3
In a sentence SET you can specify any number of values ​​for columns separated by commas:

UPDATE emp SET job = 'MANAGER', sal = sal + 1000, deptno = 20 WHERE ename = 'JONES';

UPDATE Command Example 4
In a sentence SET it is possible to specify a null value without using any special syntax (such as IS NULL). Thus, if you want to set all customer ratings from London (city = 'London') to NULL, you would enter:

UPDATE Customers SET rating = NULL WHERE city = 'London';

UPDATE Command Example 5
Explains the use of the following command syntax UPDATE:

• Both forms of proposal SET together in one statement.
• Subquery.
• A WHERE clause that limits the range of rows to be modified.

UPDATE emp a SET deptno=
(SELECT deptno FROM dept WHERE loc = 'BOSTON'), (sal, comm) = ( SELECT 1.1*AVG(sal), 1.5*AVG(comm) FROM emp b WHERE a.deptno = b.deptno) WHERE deptno IN ( SELECT deptno FROM dept WHERE loc = 'DALLAS' OR loc = 'DETROIT');

The above statement UPDATE performs the following operations:

• Modifies only those employees who work in Dallas or Detroit
• Sets the value of the deptno column for employees from Boston
• Sets the salary of each employee to 1.1 times the average salary of the entire department
• Sets the commission of each employee to 1.5 times the average commission of the entire department

Data manipulation operators (DML language).

DML statements operate on the database and are used to modify the data and retrieve the required information.

SELECT - a selection of rows that meet the given conditions. The operator implements, in particular, such relational algebra operations as "selection" and "projection".

UPDATE - changing the values ​​of certain fields in the rows of the table that satisfy the specified conditions.

INSERT - insert new rows into the table.

DELETE - delete table rows that meet specified conditions. The implementation of this operator takes into account the principles of maintaining integrity, so it may not always be executed correctly.

12.2 Interactive SQL Mode (Interactive SQL)

The corresponding mode provides for the direct work of the user with the database according to the following algorithm: using an application program (client application) or a standard utility included in the DBMS, the user:

establishes a connection to the database (confirming the availability of access rights);

· enters the corresponding SQL statement, if necessary, enters additional information in the dialog mode;

Initiates command execution.

The request text is sent to the DBMS, which:

Performs parsing of the request (checks whether the request is correct);

checks whether the user has the right to execute such a request (for example, a user who has read-only rights is trying to delete something);

selects how to execute the query - query execution plan;

Executes the request

The result of the execution is sent to the user.

Scheme of interaction between the user and the DBMS using interactive SQL

shown in fig. 12.1.

Rice. 12.1. Scheme of how interactive SQL works

12.3. Using SQL to select information from a table

Data is retrieved using the SELECT statement, which is the most commonly used statement in the SQL language. The syntax of the SELECT statement is as follows:

SELECT<список атрибутов>/*

FROM<список таблиц>

Elements that may be missing in the query are indicated in square brackets.

The ALL keyword means that the result will be all rows that satisfy the query condition, including identical rows.

DISTINCT means that duplicate rows are not included in the result set. Next comes the list of source table attributes that will be included in the result table. The * symbol means that all attributes of the source table are included in the result table.

The required keyword is FROM followed by the names of the tables being queried.

The clause with the keyword WHERE specifies conditions for selecting table rows. The result table includes only those rows for which the condition specified in the WHERE clause evaluates to true.

The ORDER BY keyword specifies the operation of ordering the rows of the result table according to the specified list of attributes.

The GROUP BY clause specifies a list of grouping attributes (this and the following keyword will be explained a bit later).

The HAVING clause specifies the conditions that apply to each group.

Separately, we note that the keywords FROM, WHERE ORDER BY are used in a similar way in other operators for manipulating data in the SQL language.

Consider the implementation of queries for a specific example presented in Lecture 8 (see Fig.

SQL - UPDATE Query

Give a list of all students.

SELECT id_st, surname

Note that if we add the ORDER BY surname clause to this query, the list will be ordered by last name. The default is to sort in ascending order. If descending ordering is required, the word DESC is added after the attribute name.

Give a list of grades that the student with the code "1" received.

SELECT id_st, mark

Issue a list of codes of students who received at least one two or three in the exams.

In the WHERE clause, you can write an expression using arithmetic comparison operators (<, >, etc.) and logical operators (AND, OR, NOT) as in conventional programming languages.

SELECT id_st, mark

WHERE (MARK >= 2) AND (MARK<= 3)

Along with comparison operators and logical operators for composing conditions in SQL (due to the specifics of the application area), there are a number of special operators that, as a rule, have no analogues in other languages. These are the operators:

IN - entry into a certain set of values;

BETWEEN - entry into a certain range of values;

LIKE - check for a match with the sample;

IS NULL - check for an undefined value.

The IN operator is used to test for a set of values. Yes, request

SELECT id_st, mark

WHERE mark IN(2,3)

gives the same result as the above query (will display the identifiers of all applicants who received at least one two or three in the exams).

The same result can be achieved using the BETWEEN operator:

SELECT id_st, mark

WHERE mark BETWEEN 2 AND 3

List all students whose last names begin with the letter A.

In this case, it is convenient to use the LIKE operator.

The LIKE operator applies exclusively to character fields and allows you to determine whether the value of a field matches a pattern. The pattern may contain special characters:

_ (underscore) - replaces any single character;

% (percent sign) - Replaces a sequence of any number of characters.

SELECT id_st, surname

WHERE surname LIKE 'A%'

Very often there is a need to calculate the minimum, maximum or average values ​​in columns. So, for example, you might want to calculate the average score. To perform such calculations, SQL provides special aggregate functions:

MIN is the minimum value in the column;

MAX - the maximum value in the column;

SUM is the sum of the values ​​in the column;

AVG is the average value in the column;

COUNT - the number of non-NULL values ​​in the column.

The following query calculates the average of all scores obtained by students in exams.

SELECT AVG(mark)

Of course, you can use aggregate functions in conjunction with the WHERE clause:

SELECT AVG(mark)

WHERE id_st = 100

This query will calculate the GPA of the student with code 100 from all the exams they have passed.

SELECT AVG(mark)

WHERE id_ex = 10

This query will calculate the average score of students based on the results of passing the exam with code 10.

In addition to the considered mechanisms, the SQL language provides a powerful apparatus for calculating aggregate functions not for the entire table of query results, but for different values ​​by groups. To do this, SQL has a special GROUP BY construct designed to specify the column by whose values ​​the grouping will be performed. So, for example, we can calculate the average score for all exams for each student. To do this, just run the following query:

SELECT id_st, AVG(mark)

All this, as usual, can be combined with the WHERE clause. At the same time, without going into the intricacies of executing a query inside the DBMS, we can assume that, first, a selection is made of those table rows that satisfy the conditions from the WHERE clause, and then grouping and aggregation are performed.

Here is a query that calculates the average score for the marks obtained in the exam with code 100 for each student.

SELECT id_st, AVG(mark)

WHERE id_ex = 100

Note that grouping can be done by more than one field.

For queries containing a GROUP BY clause, there is an important limitation: such queries can include as a result the columns by which the grouping is performed, and the columns that contain the actual results of the aggregation.

In order to format the output, there are various SQL features. For example, it is acceptable to include text in a query. Let's look at an example of how this is done:

SELECT 'Average=', AVG(mark)

WHERE id_ex = 10

As a result of this request, the user will see not just some number, but a number accompanied by explanatory text.

12.4. Using SQL to select information from multiple tables

So far, we have considered selecting information from a single table. You can query information from multiple tables by implementing the relational operations described in the corresponding section of the tutorial. It is worth mentioning that a full discussion of the topic is beyond the scope of this tutorial. This issue can be studied in detail using, for example,. Let's look at some examples of how this is done.

As a rule, in those cases when it becomes necessary to select information from different tables, they are somehow related to each other, for example, one-to-many or one-to-one relationships for some field.

Once again, let's return to the example from lecture 8. Consider the corresponding ER diagram (Fig. 12.2.).

Rice. 12.2. Example of linked tables

This example also has related tables. Consider the student, mark_st, and exam_st tables.

The mark_st table is related to the exam_st table by the id_ex field.

The mark_st table is related to the student table by the id_st field.

Let's say you want to print a list of students with the grades they got in exams. To do this, you need to run the following query:

SELECT student.surname, mark_st.id_ex, mark_st.mark

FROM student, mark_st

WHERE student.id_st = mark_st.id_st

Note the following changes compared to single table queries.

1. Two tables are listed in the FROM clause.

2. Since there are more than one tables, there is some ambiguity when referring to fields. So, in many cases it is not known from which table from the FROM list to take the field. To eliminate ambiguity, field names are specified with a prefix - the name of the table. The table name is separated from the field name by a dot.

3. The WHERE clause specifies the table join condition.

It is easy to see that the use of table name prefixes greatly clutters up the query. To avoid this clutter, aliases are used. So, you can rewrite the previous query as follows:

SELECT E.surname, M.id_ex, M.mark

FROM student E, mark_st M

WHERE E.id_st = M.id_st

12.5. Using SQL to Insert, Edit, and Delete Data
in tables

To add data to a table, the SQL standard provides the INSERT command.

Let's look at some sample queries.

INSERT INTO mark_st

VALUES (1, 2, 5)

This query inserts a row into the mark_st table containing the values ​​listed in the VALUES list. If you do not need to specify the value of a field, you can assign NULL to it:

INSERT INTO mark

VALUES (1, 2, NULL)

If it is necessary to use default values ​​for some fields, SQL allows you to explicitly specify which fields need to be filled with specific data, and which ones with default values:

INSERT INTO mark_st (id_st, id_ex)

To remove data from a table, there is a DELETE command:

This query removes all data from the student table.

You can limit the range of information to be removed as follows:

WHERE surname > 'AND'

The UPDATE command is used to update the data.

SET mark = '5'

WHERE id_st = 100 AND id_ex = 10

With this request, the grade of a student with code 100 in exam code 10 will change to "5".

12.5. SQL Language and Relational Algebra Operations

The SQL language is a means of expressing the powerful mathematical apparatus of set theory and relational algebra. This section discusses the relationship of SQL language operators with operations of relational algebra and set theory.

Union operation

Using the SQL language, the union operation is represented as follows:

Difference operation

Using the SQL language, the difference operation is represented as follows:

Projection operation

SELECT Field i1 , … , Field in

Sampling (selection) operation

WHERE( )

Intersection operation

Previous78910111213141516171819202122Next

4.6.5.DML: Data modification commands.

This group includes operators for adding, modifying and deleting records. INSERT INTO<имя_таблицы> [ (<имя_столбца>,<имя_столбца>,…)] VALUES (<значение>,<значение>,..) The list of columns in this command is optional. In this case, the values ​​for all fields of the table must be specified in the order in which these columns were listed in the command, for example: Example with a list of columns: UPDATE<имя_таблицы>SET<имя_столбца>=<значение>,… If a keyword and a condition are specified, then the command applies only to the records for which it is executed. If the condition is not specified, applies to all records. Example: Boolean expressions over constants and fields are used as a condition. The conditions allow:

• comparison operations: . In SQL, these operations can be applied not only to numeric values, but also to strings (meaning earlier and later in alphabetical order) and dates (earlier and later in chronological order).
• field check operations for value:
• range checking operations: i.
• list checking operations: and
• substring check operations: and
• Separate operations are connected by links and grouped using parentheses.

All of these keywords will be described and illustrated in detail in the paragraph on the operator. Here we restrict ourselves to giving a simple example: This command finds in the table publishers all null column values url and replaces them with the string "url not defined". DELETE FROM<имя_таблицы>[ WHERE<условие> ] All records that meet the specified condition are removed. If the keyword and condition are missing, all records are removed from the table. Example: This command removes the Super Computer Publishing entry.

4.6.6.DML: Data selection.

To retrieve records from tables in SQL, the operator is defined SELECT. With the help of this command, not only the operation of relational algebra "selection" (horizontal subset) is carried out, but also a preliminary connection (join) of two or more tables. This is the most complex and powerful SQL facility, full statement syntax SELECT looks like: SELECT<список_выбора>FROM<имя_таблицы>, … [ WHERE<условие>][ GROUP BY<имя_столбца>,… ] [ HAVING<условие> ] ,… ] Order of sentences in a statement SELECT must be strictly followed (for example, it must always precede), otherwise it will introduce errors.

We will start reviewing SELECT from its simplest forms. All of the examples below for the publications database can be run on your own from this page, so query results are not shown here.

This statement always starts with a keyword. The construct defines the column or columns to include in the result. It can consist of the names of one or more columns, or a single character (asterisk) that identifies all columns. List items are separated by commas.

get a list of all fields in a table authors: In the case when we are not interested in all records, but only those that satisfy a certain condition, this condition can be specified after the keyword.

For example, let's find all books published after 1996: Suppose now that we need to find all publications for the interval 1995 - 1997. This condition can be written in the form: Another version of this command can be obtained using the logical operation of checking for occurrence in the interval: When using the construction, all lines that are not included in the specified range are found.

Another variant of this command can be built using the logical check for list entry:

Here we explicitly set the list of values ​​we are interested in. The construction allows you to find strings that do not satisfy the conditions listed in the list.

The benefits of the keyword are most apparent in nested queries, also known as subqueries. Let's say we want to find all the titles published by Oracle Press. The names of publishing companies are contained in the table publishers, titles of books in the table titles. The keyword allows you to join both tables (without getting a common relation) and extract the necessary information in the process:

When executing this command, the DBMS first processes the nested query on the table publishers, and then passes its result to the input of the main query on the table titles.

Some problems cannot be solved using only comparison operators. For example, we want to find the website of the Wiley publishing house, but we don't know its exact name. The keyword is intended to solve this problem, its syntax is as follows:

The pattern is enclosed in quotation marks and must contain the substring pattern to be searched for. Typically, two characters are used in patterns:

• % (percent sign) - replaces any number of characters
• _ (underscore) - replaces a single character.

Let's try to find the required web-site: In accordance with the template, the DBMS will find all lines that include the substring "Wiley". Another example: find all books whose title starts with the word "SQL":<ключевой_символ>. The literal following the key character in the pattern is treated as a normal character, all subsequent characters have a normal meaning. For example, we need to find a link to a web page that is known to contain the substring "my_works" in its url: In conclusion, note that when the operator is executed, the resulting relation can have several records with the same values ​​for all fields. A keyword is used to exclude duplicate entries from the selection. The keyword specifies that all rows should be included in the result.

4.6.7.DML: Select from multiple tables.

Very often there is a situation when data selection should be made from the relation which is result of merge (join) of two other relations. For example, we need to get from the database publications information about all printed publications in the form of the following table: To do this, the DBMS must first merge the tables titles And publishers, and only then to sample from the resulting ratio.

To perform an operation of this kind, the statement after the keyword specifies a list of tables by which data is searched. After the keyword, the condition under which the merge is performed is indicated. In order to execute this request, you need to issue the command:

And here is an example where both merge and selection conditions are specified at the same time (the result of the previous query is limited to editions after 1996): You should pay attention to the fact that when different tables contain fields of the same name, then to disambiguate, the field name is preceded by the table name and sign "." (dot). (A good rule of thumb is to always specify the table name!)

Naturally, it is possible to merge more than two tables. For example, to supplement the selection described above with the names of authors of books, you need to compose an operator of the following form:

4.6.8.DML: Calculations inside SELECT.

allows you to perform various arithmetic operations on the columns of the resulting relation. In design<список_выбора>you can use constants, functions and their combinations with arithmetic operations and brackets. For example, to find out how many years have passed since 1992 (the year the SQL-92 standard was adopted) before the publication of a particular book, you can execute the command: Addition (+), subtraction (-), division (/), multiplication ( *), as well as various functions (COS, SIN, ABS - absolute value, etc.).

You can also add a string constant to the query: The so-called aggregate functions are also defined, which perform operations on a set of identical fields in a group of records. Among them:

• AVG(<имя поля>) - average over all values ​​of this field
• COUNT(<имя поля>) or COUNT(*)- number of records
• MAX(<имя поля>) - the maximum of all values ​​​​of this field
• MIN(<имя поля>) - the minimum of all values ​​​​of this field
• SUM(<имя поля>) — the sum of all values ​​of this field

Note that each aggregate function returns the only thing meaning. Examples: determine the date of publication of the most "oldest" book in our database count the number of books in our database: The scope of the function data can be limited using a boolean condition. For example, the number of books that have the word "SQL" in their titles:

4.6.9.DML: Data grouping.

The grouping of data in the statement is carried out using a keyword and a keyword that sets the conditions for dividing records into groups.

is inextricably linked with aggregation functions; without them, it is practically not used. divides the table into groups, and the aggregate function calculates the total value for each of them. For example, let's determine the number of books of each publisher in our database:

The keyword works as follows: first, it splits the strings into groups, then conditions are imposed on the resulting sets. For example, let's remove from the previous query those publishers that have only one book: Another use case is to include in the result only those publishers whose name ends with the substring "Press": What is the difference between these two use cases? In the second variant, we could place the record selection condition in the keyword section, but in the first variant, this cannot be done, since it does not allow the use of aggregating functions.

4.6.10.DML: Data sorting.

The keyword is used to sort the data received by the operator. With it, you can sort the results by any column or expression specified in<списке_выбора>. Data can be sorted in ascending or descending order. Example: Sort a list of authors alphabetically: A more complex example: get a list of authors sorted alphabetically and a list of their publications, with each author sorting the list of books by publication date in reverse order (i.e., most recent books first, then more and more "ancient"): The keyword here specifies the reverse order of sorting by field yearpub, keyword (it can be omitted) - direct sort order by field author.

4.6.11.DML: Merge operation.

SQL provides the ability to perform the UNION relational algebra operation on relations that are the results of a SELECT statement. Naturally, these relationships must be defined according to the same schema. Example: get all Internet links stored in the database publications. These links are stored in tables publishers And wwwsites. In order to get them in one table, we must build the following query:

4.6.12 Using views.

So far we have been talking about tables really are stored in the database. These are the so-called base tables. There is another kind of table called "views" (sometimes referred to as "view tables").

When the contents of the underlying tables change, the DBMS automatically re-executes the queries that create the view, resulting in the corresponding changes in the views.

The view is defined with the command

CREATE VIEW<имя_представления> [<имя_столбца>,…] AS<запрос> The following restrictions must be observed:

• the view must be based on a single query (not allowed)
• the output of the query generating the view must be unordered (not allowed)

Let's create a view that stores information about authors, their books and publishers of these books: Now any user whose access rights to this view are sufficient can select data from books. For example: (User rights to access views are also assigned using commands.)

From the above example, the meaning of using views is clear enough. If queries like "select all books by this author with publishers" are performed often enough, then the creation of a table books will significantly reduce the overhead of joining four base tables authors, titles, publishers And titleauthors. In addition, information can be presented in a view that is not explicitly stored in any of the underlying tables. For example, one of the view's columns could be computed:

Here, one more, not previously described, possibility is used - assigning new names to the columns of the view.

In the example above, the number of publications made by each publisher will be stored in a column named books_count. Note that if we want to assign new names to the columns of the view, we need to specify names for all columns. The data type of a view column and its null status always depend on how it was defined in the underlying table(s).

A query to retrieve data to a view looks exactly like a query to any other table. However, there are restrictions on changing data in a view. Briefly about them, we can say the following:

• If a view is based on a single table, changes to the data in it are allowed. This changes the data in the associated table.
• If the view is based on more than one table, then data changes in it are not allowed, because in most cases, the DBMS cannot correctly recover the base table schema from the view schema.

Removing a view is done using the operator:

DROP VIEW<имя_представления>

4.6.13. Other features of SQL.

The features described below have not yet been standardized, but are present to one degree or another in almost all modern DBMSs.

• Stored procedures. Practical experience in creating data processing applications shows that a number of data operations that implement common logic for all users and are not related to the user interface should be transferred to the server. However, to write procedures that implement these operations, standard features are not enough, since operators for processing branches, loops, etc. are needed here. Therefore, many DBMS vendors offer their own procedural extensions (Oracle's PL/SQL, etc.). These extensions contain logical statements (IF ... THEN ... ELSE), conditional jump statements (SWITCH ... CASE ...), loop statements (FOR, WHILE, UNTIL) and control transfer statements to procedures (CALL, RETURN). These tools create functional modules that are stored on the server along with the database. These modules are usually called stored procedures. They can be called with parameter passing by any user who has the appropriate rights to do so. In some systems, stored procedures can also be implemented as modules external to the DBMS in general-purpose languages, such as C or Pascal.

  SQL - UPDATE query

  Example for PostgreSQL DBMS: CREATE FUNCTION<имя_функции> ([<тип_параметра1>,…<тип_параметра2>]) RETURNS <return types> AS [ | <имя_объектного_модуля>]LANGUAGE ‘SQL’ | 'C' | ‘internal’ The created function is called from the SELECT statement (same as aggregation functions are called). For more information about stored procedures, see E. Eisenberg's article New Standard for Stored Procedures in SQL, DBMS N 5-6, 1996.

• triggers. For each table, a stored procedure without parameters can be assigned, which is called when executing the modification statement for this table (INSERT, UPDATE, DELETE). Such stored procedures are called triggers. Triggers are executed automatically, regardless of whether it is the action of a human operator or an application program that causes the data modification. "Average" syntax of the trigger creation statement: CREATE TRIGGER<имя_триггера>ON<имя_таблицы>FOR ( INSERT | UPDATE | DELETE ) [, INSERT | UPDATE | DELETE] ... AS The keyword specifies the name of the table for which the trigger is defined, the keyword specifies which data modification command(s) activates the trigger. The statements after the keyword describe the actions that the trigger performs and the conditions for performing these actions. Any number of statements, stored procedure calls, and so on can be listed here. The use of triggers is very convenient for performing integrity constraint check operations (see chapter 4.3).
• Event monitors. A number of DBMSs allow the creation of such stored procedures that continuously scan one or more tables in order to detect certain events (for example, the average value of a column reaches a specified limit). In the event of an event, a trigger, stored procedure, external module, etc. can be initiated. Example: Let our database be part of an automated process control system. In the field of one of the tables, the readings of the temperature sensor installed on the cutter of the lathe are entered. When this value exceeds the specified limit, an external program is launched that changes the parameters of the machine.

The question is old, but I felt that the best answer has not yet been given.

Is there a syntax for UPDATE... without specifying column names ?

General solution with dynamic SQL

You don't need to know the column names, except for some unique columns to join (in the example). Works well for every possible corner case I can think of.

This is specific to PostgreSQL. I'm building dynamic code based on schema_information, specifically a table that is defined in ANSI SQL and supports most modern DBMSs (except Oracle). But a statement with PL/pgSQL code executing dynamic SQL is completely non-standard PostgreSQL syntax.

SQL UPDATE

Assuming the corresponding column in for everyone column in, but not vice versa.

may have additional columns.

is optional to update only the selected row.

SQL Fiddle.

Similar answers with more explanation:

Partial solutions with plain SQL

With a list of separable columns

You still need to know the list of column names that both tables share. With syntax shortcut to update multiple columns - shorter than other answers suggested so far anyway.

SQL Fiddle.

This syntax was introduced with Postgres 8.2 in December 2006, long before the question was asked.
More details are in the manual and the corresponding answer on dba.SE:

With a list of columns in

If all columns are defined (but not required),
And You you know column names (but not required).

is appended to the string from where all columns with the same name have the same value. We don't need to update in this case (nothing changes) and can remove these lines at the beginning of the process ().
We still need to find the matching string, so in the outer query.

SQL Fiddle.

This is standard SQL, except for the clause.
It works no matter which of the columns is actually present in, but the query cannot distinguish between actual NULL values ​​and missing columns in, so it is only reliable if all columns in are defined.

There are several options, depending on what you you know about both tables.

I have two tables:

will always be a subset (which means all columns are also in).

I want to update a record with a certain in with their data from for all columns This one exists in both in and in

Is there a syntax or any other way to do this without specifying the column names, just saying "set all columns A" ?

I'm using PostgreSQL so a specific non-standard command is also accepted (not recommended though).

These concepts refer to databases.

DDL - Data Definition Language. With the help of this language, data is defined by specifying the data type, representation structures. Actually, it's part of the language. SQL. But only one. These are the statements that are associated with the creation commands, for example CREATE TABLE. The result of these operations is recorded in the system catalog, which stores information about the tables.

DML - Data Manipulation Language. It is a data management language that can be used to retrieve and modify data. There are two varieties of these languages.

Procedural procedural non procedural non-procedural

The difference between them is not what it seems at first glance. For a programmer, this type has procedures, no procedures. In fact, procedural languages ​​process data sequentially. That is, record after record, and non-procedural ones operate on whole sets at once. And the difference can be seen from here that in procedural languages ​​it is indicated how to receive data, and in non-procedural languages ​​what we want to receive. The process in a non-procedural language does not concern us and it is hidden from the developer. The most common non-procedural language is SQL. And here it should become clear what it is when we indicate not the path, but the result. Operator SQL type SELECT * FROM TABLE tells about the result we want. And in this case, we want to get all the records and columns from the table.

UPDATE OPERATOR

There is another non-procedural language QBE. Let's look at it from the side SQL. So, SQL this is two parts, the first part is for creating objects in the database DDL, and the second part is for manipulating the data in these objects DML. Why such a division? Designing a database is not an easy task and requires serious study. There are special programs that help build a data structure, check connections, and resolve contradictions at the design stage. As a result of the work of these programs, a set of commands is formed DDL(in the form of operators SQL) that run on the database server and all structures are ready to go. Then the filling begins, I use it already DML, and then work, again using DML(in the form of operators SQL).

Previous Step | Next Step | Table of contents
Author Kaev Artem.