SQL Loop Through List Values

SQL, or Structured Query Language, is a powerful tool for managing and manipulating data in relational databases. One common task in SQL is looping through a list of values to perform operations such as inserting, updating, or selecting data. In this article, we will explore the different ways to loop through list values in SQL, including using loops, cursors, and set-based operations.

Introduction to SQL Loops

SQL loops are used to execute a block of code repeatedly for a specified number of times. There are several types of loops in SQL, including WHILE loops, FOR loops, and CURSOR loops. Each type of loop has its own advantages and disadvantages, and the choice of which one to use depends on the specific requirements of the task at hand.

WHILE Loops in SQL

A WHILE loop in SQL is used to execute a block of code as long as a certain condition is true. The basic syntax of a WHILE loop is as follows:

WHILE condition
BEGIN
    -- code to be executed
END

For example, to loop through a list of values and insert them into a table, you can use a WHILE loop as follows:

DECLARE @i INT = 1
DECLARE @list TABLE (id INT, value VARCHAR(50))

INSERT INTO @list (id, value)
VALUES (1, 'Value 1'), (2, 'Value 2'), (3, 'Value 3')

WHILE @i <= (SELECT MAX(id) FROM @list)
BEGIN
    INSERT INTO mytable (value)
    VALUES ((SELECT value FROM @list WHERE id = @i))

    SET @i = @i + 1
END

FOR Loops in SQL

A FOR loop in SQL is used to execute a block of code for each row in a result set. The basic syntax of a FOR loop is as follows:

FOR variable IN (SELECT statement)
LOOP
    -- code to be executed
END LOOP

For example, to loop through a list of values and insert them into a table, you can use a FOR loop as follows:

FOR @i IN (SELECT id, value FROM @list)
LOOP
    INSERT INTO mytable (value)
    VALUES (@i.value)
END LOOP

CURSOR Loops in SQL

A CURSOR loop in SQL is used to execute a block of code for each row in a result set. The basic syntax of a CURSOR loop is as follows:

DECLARE cursor_name CURSOR FOR
SELECT statement

OPEN cursor_name

FETCH NEXT FROM cursor_name INTO variable

WHILE @@FETCH_STATUS = 0
BEGIN
    -- code to be executed
    FETCH NEXT FROM cursor_name INTO variable
END

CLOSE cursor_name
DEALLOCATE cursor_name

For example, to loop through a list of values and insert them into a table, you can use a CURSOR loop as follows:

DECLARE @cursor CURSOR
DECLARE @id INT, @value VARCHAR(50)

SET @cursor = CURSOR FOR
SELECT id, value FROM @list

OPEN @cursor

FETCH NEXT FROM @cursor INTO @id, @value

WHILE @@FETCH_STATUS = 0
BEGIN
    INSERT INTO mytable (value)
    VALUES (@value)

    FETCH NEXT FROM @cursor INTO @id, @value
END

CLOSE @cursor
DEALLOCATE @cursor

Set-Based Operations in SQL

Set-based operations in SQL are used to perform operations on entire datasets at once, rather than looping through individual rows. This approach can be much faster and more efficient than looping, especially for large datasets. Some common set-based operations in SQL include SELECT, INSERT, UPDATE, and DELETE statements.

INSERT Statement

The INSERT statement in SQL is used to insert new rows into a table. The basic syntax of an INSERT statement is as follows:

INSERT INTO table_name (column1, column2,...)
VALUES (value1, value2,...)

For example, to insert a list of values into a table, you can use an INSERT statement as follows:

INSERT INTO mytable (value)
VALUES ('Value 1'), ('Value 2'), ('Value 3')

UPDATE Statement

The UPDATE statement in SQL is used to update existing rows in a table. The basic syntax of an UPDATE statement is as follows:

UPDATE table_name
SET column1 = value1, column2 = value2,...
WHERE condition

For example, to update a list of values in a table, you can use an UPDATE statement as follows:

UPDATE mytable
SET value = 'New Value'
WHERE id IN (1, 2, 3)

DELETE Statement

The DELETE statement in SQL is used to delete existing rows from a table. The basic syntax of a DELETE statement is as follows:

DELETE FROM table_name
WHERE condition

For example, to delete a list of values from a table, you can use a DELETE statement as follows:

DELETE FROM mytable
WHERE id IN (1, 2, 3)