SQL WHILE

What is SQL WHILE?

Executes a block of statements repeatedly while a Boolean expression remains TRUE.

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Compatible dialects for SQL WHILE:

SQL WHILE Full Explanation

SQL WHILE is an imperative control-flow construct that lets you create iterative loops in procedural extensions of SQL. On each iteration the Boolean condition is evaluated; if it returns TRUE the loop body runs, then control returns to the test. When the condition becomes FALSE the loop exits and execution continues with the statement that follows the WHILE block. WHILE is not part of the ANSI-SQL standard but is available in several dialects such as T-SQL (SQL Server and Sybase) and within stored-program languages in MySQL, PostgreSQL, and Oracle. Companion keywords BREAK and CONTINUE (or LEAVE/ITERATE) allow early exit or skipping to the next iteration. Because the same query can be executed set-based, overusing WHILE can hurt performance. Use it only when row-by-row logic, complex procedural branching, or retry logic is unavoidable.

SQL WHILE Syntax

-- T-SQL general form
WHILE Boolean_expression
BEGIN
    -- statements
END;

-- MySQL stored procedure form
WHILE Boolean_expression DO
    -- statements
END WHILE;

SQL WHILE Parameters

condition (Boolean expression) - Evaluated before each iteration.
statement_block (statement or BEGIN...END block) - Code that runs while the condition is TRUE.

Example Queries Using SQL WHILE

-- Example 1: Basic counter loop in SQL Server
DECLARE @counter INT = 1;
WHILE @counter <= 5
BEGIN
    PRINT CONCAT('Counter = ', @counter);
    SET @counter += 1;
END;

-- Example 2: Retry logic with BREAK
DECLARE @attempts INT = 1;
WHILE @attempts <= 3
BEGIN
    BEGIN TRY
        EXEC usp_ProcessBatch;
        BREAK; -- exit on success
    END TRY
    BEGIN CATCH
        PRINT 'Retrying...';
        SET @attempts += 1;
    END CATCH
END;

Expected Output Using SQL WHILE

The first example prints Counter = 1 through Counter = 5 in the Messages tab
The second example keeps calling usp_ProcessBatch until it succeeds or three attempts have been made

Use Cases with SQL WHILE

Implementing retry logic when calling stored procedures or external resources
Performing row-by-row operations that cannot be expressed set-based
Building dynamic batches where the iteration count is not known in advance
Simulating cursors in lightweight scenarios

Common Mistakes with SQL WHILE

Forgetting to update the loop variable, causing infinite loops
Using WHILE for tasks that could be done with a single set-based statement, leading to poor performance
Assuming WHILE is available in every SQL dialect; it is not supported in SQLite and requires procedural blocks in MySQL/PostgreSQL
Misplacing BREAK or CONTINUE so logic flow is unintentionally altered

First Introduced In

Sybase SQL Server 4.x (early T-SQL)

Frequently Asked Questions

What does SQL WHILE do?

It evaluates a condition and runs a statement block repeatedly while that condition stays TRUE.

Can I use WHILE in MySQL?

Yes, but only inside stored procedures, functions, or triggers. The syntax uses WHILE ... DO ... END WHILE.

How do I prevent infinite loops?

Always change the loop variable or include logic that eventually makes the condition FALSE.

What alternatives exist to WHILE loops?

Set-based UPDATE or INSERT statements, window functions, and cursors can often replace explicit loops.