WHILE loops are a fundamental construct in T-SQL (and in programming in general) for executing repetitive tasks. Within these loops, the CONTINUE keyword serves as a powerful control flow tool, allowing developers to skip iterations and optimise loop execution.

This article explores the usage and benefits of the CONTINUE keyword in SQL Server WHILE loops.

What is the CONTINUE Keyword?

The CONTINUE keyword is an optional argument that can be used within a WHILE loop to skip the rest of the current iteration and move directly to the next iteration of the loop. When encountered, it immediately jumps back to the beginning of the loop, re-evaluating the loop condition.

Syntax and Basic Usage

The basic T-SQL syntax for using CONTINUE in a WHILE loop is as follows:

WHILE condition
BEGIN
    -- Loop body
    IF some_condition
        CONTINUE
    -- Rest of the loop body
END

When the specified condition is met, the CONTINUE statement causes the loop to skip any remaining code in the current iteration and start the next iteration.

Example

Let’s look at a simple example to illustrate the use of CONTINUE:

DECLARE @counter INT = 1;

WHILE @counter <= 10
BEGIN
    IF @counter % 2 = 0
    BEGIN
        SET @counter = @counter + 1;
        CONTINUE;
    END
    
    PRINT 'Odd number: ' + CAST(@counter AS VARCHAR(2));
    SET @counter = @counter + 1;
END

Result:

Odd number: 1
Odd number: 3
Odd number: 5
Odd number: 7
Odd number: 9

In this example, the loop prints only odd numbers between 1 and 10. When an even number is encountered, the CONTINUE statement skips the PRINT statement and moves to the next iteration.

Best Practices and Considerations

• Use Sparingly: While CONTINUE is useful, overuse can make your code harder to follow. Use it judiciously to maintain code clarity.
• Combine with Comments: When using CONTINUE, consider adding comments to explain the skipping logic, especially in complex scenarios.
• Avoid Infinite Loops: Ensure that your loop’s termination condition is still reachable when using CONTINUE.
• Be Mindful of Performance: It’s always a good idea to profile your code to ensure it’s providing the expected benefits.

Conclusion

The CONTINUE keyword is a valuable tool in SQL Server WHILE loops, offering improved control flow and potential performance benefits. By understanding and appropriately using CONTINUE, developers can write more efficient and readable loop structures in their T-SQL code.