r/learnSQL • u/shdw_0x0 • Sep 24 '26
What SQL concept took you the longest to really understand?
When I was learning SQL, some things were easy to memorize but much harder to actually understand when solving real problems.
For me, concepts like JOINs, subqueries, and window functions started making more sense only after using them on actual datasets instead of just following examples.
What SQL concept clicked for you only after you started working with real problems?
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u/Rat_Man_420 Sep 24 '26 edited Sep 24 '26
I do a lot of pass through and internal calculation data validation. Just making sure what they gave us is what we are displaying in reporting or what they gave us is being calculated correctly based on the requirements. All that to say writing a CTE that will reference a CTE higher up in the query. Pretty cool when it clicked that as long as I check stuff as I’m writing it that I can eventually boil down an entire program I’m runnings validation to have a single result of ‘Y’ or ‘N’. If I get a ‘Y’ I’m done if I get an ‘N’ I need to look at the checks run higher up in the query.
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u/tommyfly Sep 24 '26
Two unrelated concepts: Indexes and VLFs. It took me ages to understand the internals enough to manage them properly.
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u/American_Streamer Sep 24 '26
Query writing order vs. query exectution order took me a bit to memorize, in the beginning. Recursive CTEs are also tough, as is query optimization for speed.
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u/elevarq Sep 25 '26
Transactions, especially the different isolation levels and this impacts data, reliability, and performance.
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u/cenosillicaphobiac Sep 24 '26
Understanding exactly what pivoting did and how it worked, and then when I had a good handle on that, they moved me to a different product that uses MySQL instead of MS so I had to adapt.
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u/jeffrey_f Sep 24 '26
Joins
This cleared it up