Why is mysql query taking a long time to complete?

Question

PHP

Hello, I'm new to building databases and querying them?
I wrote such a request, but it takes a very long time (15 sec), please explain why it takes so long to complete?

Answer 1

nested selects are not very productive ...
if I'm not mistaken, then on each such select a temporary table is created in memory, filled with results, and then the parent query passes through it. after which she is released.