Debugging stored procedures, functions, and views in SQL Server can be a challenging task, especially if you don’t have access to the debugger or if you need to troubleshoot issues in a production environment. In this article, we will discuss three major categories of debugging techniques that can help you identify and resolve problems in your SQL code.

Add PRINT and/or SELECT statements

One of the simplest and most effective ways to debug SQL code is by adding PRINT and/or SELECT statements. These statements allow you to output the values of variables, query results, and other important information during the execution of your code. By strategically placing these statements throughout your code, you can track the flow of execution, identify the values of variables, and pinpoint any issues that may arise.

For example, let’s say you have a loop that seems to be running indefinitely. By adding a PRINT statement at the end of the loop to display the values of the loop counter and the maximum counter, you can quickly identify the problem:

DECLARE @MyCounter int = 0, @MaxCounter int = 100;

WHILE @MyCounter < @MaxCounter
BEGIN
    -- Your code here
    
    PRINT 'MyCounter ' + CAST(@MyCounter as varchar(20));
    PRINT 'MaxCounter ' + CAST(@MaxCounter as varchar(20));
    
    SET @MyCounter = @MyCounter + 1;
END

By examining the output of the PRINT statements, you can determine why the loop is not terminating as expected and make the necessary adjustments to your code.

Comment/uncomment out pieces of code to isolate problems

Another useful debugging technique is to comment or uncomment sections of your code to isolate problems. This technique is particularly helpful when you need to simplify your code or identify the root cause of an issue.

For example, if you are not getting the expected results from a SELECT statement, you can start by removing parts of the WHERE clause until the desired rows are returned. By toggling back and forth between commented and uncommented conditions, you can narrow down the problem and determine whether it is a logic problem or a data problem.

SELECT name, number, type, low, high, status
FROM spt_values
WHERE low IS NOT NULL
--  AND name LIKE '%data%';

In the above example, commenting out the LIKE condition reveals the expected data. By uncommenting the LIKE condition and commenting the IS NOT NULL condition, you can further investigate the issue and determine that the problem is due to case sensitivity.

Change INSERT/UPDATE/DELETE statements to SELECTs

When debugging SQL code, it is advisable to minimize actual data changes to avoid complicating the debugging process. Instead of executing INSERT, UPDATE, or DELETE statements, you can modify them to SELECT statements to observe the data that would be affected.

For example, if you are debugging an UPDATE statement, you can comment out the original command and replace it with a SELECT statement that retrieves the columns in the WHERE clause and the column being updated:

-- UPDATE your_table
-- SET column = new_value
-- WHERE condition;

SELECT column, new_value
FROM your_table
WHERE condition;

By examining the results of the SELECT statement, you can verify whether the correct rows are being targeted and determine if the update operation is working as intended.

Other considerations

When debugging SQL code, it is important to have a clear understanding of the expected output and work in a development environment. Having representative data that closely matches the production environment can also be beneficial in identifying and resolving issues.

By utilizing these debugging techniques and following best practices, you can effectively troubleshoot and resolve problems in your SQL Server code.