WHILE (Transact-SQL)

Artikel
05/16/2013

Sets a condition for the repeated execution of an SQL statement or statement block. The statements are executed repeatedly as long as the specified condition is true. The execution of statements in the WHILE loop can be controlled from inside the loop with the BREAK and CONTINUE keywords.

Topic link icon Transact-SQL Syntax Conventions

Syntax

WHILE Boolean_expression 
     { sql_statement | statement_block | BREAK | CONTINUE }

Arguments

Boolean_expression
Is an expression that returns TRUE or FALSE. If the Boolean expression contains a SELECT statement, the SELECT statement must be enclosed in parentheses.
{sql_statement | statement_block}
Is any Transact-SQL statement or statement grouping as defined with a statement block. To define a statement block, use the control-of-flow keywords BEGIN and END.
BREAK
Causes an exit from the innermost WHILE loop. Any statements that appear after the END keyword, marking the end of the loop, are executed.
CONTINUE
Causes the WHILE loop to restart, ignoring any statements after the CONTINUE keyword.

Remarks

If two or more WHILE loops are nested, the inner BREAK exits to the next outermost loop. All the statements after the end of the inner loop run first, and then the next outermost loop restarts.

Examples

A. Using BREAK and CONTINUE with nested IF...ELSE and WHILE

In the following example, if the average list price of a product is less than $300, the WHILE loop doubles the prices and then selects the maximum price. If the maximum price is less than or equal to $500, the WHILE loop restarts and doubles the prices again. This loop continues doubling the prices until the maximum price is greater than $500, and then exits the WHILE loop and prints a message.

USE AdventureWorks2012;
GO
WHILE (SELECT AVG(ListPrice) FROM Production.Product) < $300
BEGIN
   UPDATE Production.Product
      SET ListPrice = ListPrice * 2
   SELECT MAX(ListPrice) FROM Production.Product
   IF (SELECT MAX(ListPrice) FROM Production.Product) > $500
      BREAK
   ELSE
      CONTINUE
END
PRINT 'Too much for the market to bear';

B. Using WHILE in a cursor

The following example uses @@FETCH_STATUS to control cursor activities in a WHILE loop.

DECLARE Employee_Cursor CURSOR FOR
SELECT EmployeeID, Title 
FROM AdventureWorks2012.HumanResources.Employee
WHERE JobTitle = 'Marketing Specialist';
OPEN Employee_Cursor;
FETCH NEXT FROM Employee_Cursor;
WHILE @@FETCH_STATUS = 0
   BEGIN
      FETCH NEXT FROM Employee_Cursor;
   END;
CLOSE Employee_Cursor;
DEALLOCATE Employee_Cursor;
GO

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