Nested SQL While Loop

The Nested SQL While Loop is nothing but placing While Loop inside another While Loop. The Nested While loops are instrumental when working with multi-layered data. Because when we want to select the layered data, we have to use this Nested While loop to extract the layered data. But please be careful while using it.

Nested While Loop Syntax

The syntax of the Sql Server Nested While Loop in is as shown below.

WHILE Expression
	BEGIN
		WHILE @Val2 <= 10
			BEGIN
				-- Second While Loop Statements
				Statement 1
				Statement 2
				...........
				Statement N
			END
		-- Below Statements are Outside the Second While Loop
		-- First While Loop Statements
		Statement 1
		Statement 2
		...........
		Statement N
	END
-- This Statement is Outside the First While Loop

If you observe the above Nested While Loop syntax, We placed the While loop inside another one. We already explained the While loop syntax in our previous article.

Step 1: First, it checks for the condition inside the first While loop.

  • If the expression result is True, the begin and end code block inside the While loop will execute. Next, it will start the second While loop. Go to Step 2
  • If the result is False, it will exit from While Loop

Step 2: It will verify the condition in the Nested SQL While Loop (second While loop).

  • If the result is True, the code inside the second While loop begin…end will execute. It means the SQL Server runs statements from Statement 1 to N.
  • If it is False, it exits from the second While Loop.

3rd Step: Once it exits from the second While loop, it will check for the condition inside the first While loop (repeating Step 1 )

Nested SQL While Loop Example

This Nested While loop program will print the Multiplication table of 1 and 2 up to 10. For this, we will nest one While loop inside another called a nested one.

--Nested SQL While Loop Example

DECLARE @Val1 INT,
	@Val2 INT
SET @Val1 = 1

WHILE @Val1 <= 2
	BEGIN
		SET @Val2 = 1
		WHILE @Val2 <= 10
			BEGIN
				PRINT CONVERT(VARCHAR, @Val1) + ' * ' + CONVERT(VARCHAR, @Val2) + 
					' = ' + CONVERT(VARCHAR, @Val1 * @Val2)
				SET @Val2 = @Val2 + 1
			END
		SET @Val1 = @Val1 + 1
	END
Nested SQL While Loop 1

Within this SQL Nested While Loop example, we first created two variables called Val1 and Val2 and then initialized the @Val1 to 1 using the following statement.

DECLARE @Val1 INT,
	@Val2 INT
SET @Val1 = 1

In the next line, we used the SQL Server while loop with expression. If the expression result is true, it will enter to Nested While loop.

WHILE @Val1 <= 2
BEGIN
	SET @Val2 = 1
	WHILE @Val2 <= 10
		BEGIN
			PRINT CONVERT(VARCHAR, @Val1) + ' * ' + CONVERT(VARCHAR, @Val2) + 
				' = ' + CONVERT(VARCHAR, @Val1 * @Val2)
			SET @Val2 = @Val2 + 1
		END
	SET @Val1 = @Val1 + 1
END

From the above screenshot, you can observe that this Nested SQL While Loop query prints multiplication table for 1 and 2.

First While Loop First Iteration

In the first While loop, @Val1 is initialized to 1, and then it will check whether @Val1 is less than or equal to 2. This condition (1 <= 2) is True, so it will enter into a second While loop

Nested While Loop or Second While Loop First Iteration

In the second While loop, @Val2 is initialized to 1, and check whether @Val2 is less than or equal to 10. This condition is True. So, the statements inside the second While loop will execute

PRINT CONVERT(VARCHAR, @Val1) + ' * ' + CONVERT(VARCHAR, @Val2) + 
      ' = ' + CONVERT(VARCHAR, @Val1 * @Val2)

@Val1 * @Val2 ==> 1 * 1 = 1

Next, the @Val2 value will be incremented by 1 (SET @Val2 = @Val2 + 1). Please refer Arithmetic Operators article to understand the + notation.

Second While Loop Second Iteration

Here, @Val2 is incremented by one. So, @Val2 =2. It will check whether @Val2 is less than or equal to 10. This condition (2 <= 10) is True. So, it will execute the statements inside the second While loop

@Val1 * @Val2 ==> 1 * 2 = 2
Next, the @Val2 value will increment by 1.

This process will repeat until @Val2 reaches 11. Once the condition inside the SQL nested While loop fails, the compiler will exit from the second While loop, and the @Val1 value will increment by 1 (SET @Val1 = @Val1 + 1).

First While Loop Second Iteration

@Val1 = 2. Check whether @Val1 is less than or equal to 2. This condition ( 2 <= 2) is True, so it enters into a second While loop

Second While Loop First Iteration

Within the second While loop, @Val2 will initialize to value 1 and check whether @Val2 is less than or equal to 10. This condition is True, so it executes the statements inside the second While loop

@Val1 * @Val2 ==> 2 * 1 = 2

Next, the @Val2 value will increment by 1 (SET @Val2 = @Val2 + 1).

Second While Loop Second Iteration

Here, @Val2 will increment by 1, so @Val2 = 2. Next, it will check whether @Val2 is less than or equal to 10. The condition (2 <= 10) is True, so it will start executing the statements inside the second While loop
@Val1 * @Val2 ==> 2 * 2 = 4
Next, the @Val2 value will increment by 1 (SET @Val2 = @Val2 + 1)

This process will repeat until @Val2 reaches 11. Once it reaches 11, the condition (11 <= 10) inside the second While loop fails. So, it will exit from the nested While loop, and the @Val1 value will increment by 1 (SET @Val1 = @Val1 + 1).

First While Loop Third Iteration:

Here, i = 3 means the expression (@Val1 <= 2) will be False. So, While loop will terminate, remember, no need to check the second loop.

Categories SQL