I often need to do the same task as you, working with someone else's SQL statement, usually to create something similar. I like to copy the original SQL statement into a New Query window and make updates to the copy. Since in this case there are many unions and sub-queries, I would split each section of the statement at the union or sub-query clause and verify that each part worked correctly on its own. I sometimes include PRINT statements between the parts. If one of the statements fails, at least some of the PRINT statements and SELECT statements would show their output before the failure. Then I could narrow my checking to the first statements below the working parts.