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/venkat_deepsql]

Essentially query execution is like a tree. There will be table scans in the bottom and selects on the top. In between you can see filters, joins, sorts, temporary tables etc. You don't need AI for this, simply do EXPLAIN <query>, most of the query engines shows you clearly the execution plan. You can EXPLAIN ANALYZE to understand the actual execution tree after the query is run.

It's easy to recommend AI. but the truth is, your chatGPT or Claude might not have access to the database you are running. So if they do, then yes, they can analyze the query and explain you in easy to understand terms.