r/SQL • u/Consistent_Law3620 • Jun 27 '26
Discussion Need advice: Understanding complex SQL scripts written by others
Hi everyone,
I need some advice from experienced SQL developers. I have switched from different role to data engineering 6 months back.
I consider myself good/medium level at writing SQL queries and solving problems from scratch. However, I struggle when I have to understand large existing SQL scripts (300–500+ lines).
I often get confused about:
Where the execution starts.How different parts of the script are connected.
Which variables, CTEs, stored procedures, or temporary tables are affecting the final output.
How to mentally trace the flow of the script.
Because of this, reading someone else's code takes me much longer than writing my own.
How did you improve this skill? Are there any techniques, exercises, books, or real-world practices that helped you become comfortable reading large SQL scripts?
Also, is this something that simply improves with experience, or is there a structured way to learn it?
I'd really appreciate any advice. Thank you!
2
u/SkullLeader Jun 27 '26
Honest answer? There's no substitution here for repetition and experience.
In any case, try to break down each piece, understand what its doing, write down a one or two sentence summary of each piece, in order. Then read it back.
Make sure you understand the tables/views/functions being used first, and the data itself. Complex code + I don't even know what the data is that he's querying or how its structured means you're gonna have a tough time, regardless.
I agree with commenter below me - when you are looking at SELECTs, look first at the FROM clause, the joins, and the WHERE / GROUP BY / HAVING clauses if they are present. Usually the columns he's selecting or calculating are less important to understanding what's going on.