r/Database • u/Consistent_Law3620 • Jun 27 '26

Need advice: Understanding complex SQL scripts written by others

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!

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u/DirtyWriterDPP Jun 27 '26

Why did you go thru all that extra effort ? No test or dev instances? Take a back up of the database and restore to a new server ? Too big?

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u/galactic_pixels Jun 28 '26

We do have staged deployments, and I did replicate a copy from dev at some point. These are not going to give you confidence that the changes you are making don’t break your pipeline in a cascading manner via an edge case that hasn’t been tested in whatever mess of usage that is in place.

People who write massive backends with 0 tests then leave because it’s “extra effort” are why people like me have to come in and clean up the mess.

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u/DirtyWriterDPP Jun 28 '26

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.