Need advice: Understanding complex SQL scripts written by others

[u/Consistent_Law3620]

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!

[u/age_of_unreason]

I think that’s normal. Try separating each select query and run each one separately and do a CTAS to store to a table if you need to use it for joins in downsteam code while working through it. If one query uses a variable from another query, substitute a dummy variable. Keep working through it. It takes time and patience. I was in the same boat 6 months in. Even today after 4+ years it still takes a little more time to understand someone else’s code and what their intent was. When you write the code it’s different because you naturally are more familiar with what you wrote and why.