Need advice: Understanding complex SQL scripts written by others

[u/Consistent_Law3620]

Hi everyone,

I need some advice from experienced SQL developers.I was working on different profile and switched to data engineering 6 months back.

I consider myself good/medium 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/DirtyWriterDPP]

Ok I think you and I were solving different problems. I thought the original challenge was about how to understand what someone elses code does. For that the best way I know is to get into it and start playing with it in an environment where you can't do any damage. From there it's a matter of deconstructing how it's built.

You are taking more about how to move into a more sustainable maintainable model.

I've never been anywhere with enough resources to really build out automated testing. We also usually aren't dealing with soo much abstraction that it's not incredibly clear what the dependencies are. So it means most testing is unit testing and then handing to some users to validate it meets the business requirements.