MySql optimization example: modify the cross join method to subquery method in order to change the execution plan

source of problem

The content of the question comes from the question and answer: https://ask.csdn.net/questions/7898705.

This is a small problem caused by a beginner who inherited garbage code from an irresponsible predecessor.

Then, Lao Gu gave a simple answer in the question and answer. Unexpectedly, this was the beginning of suffering. I was asked by my friends in the short section of the website.

CSDN illiterate Lao Gu's blog ,