(Original) knowledge about SQL statements

Differences between in and exists:
All the content in the in traversal;
Exists ends when the conditions are met;
Therefore, the efficiency of using exists is higher than that of using in;
However, this is not absolute. When in and exists are of the same efficiency, they are used to exists;

Use of where:
The order of where conditions varies greatly with the efficiency;

Difference between Union and Union all:
Union performs two operations.
1. Put the two result sets together;
2. perform distinct on the combined result set.
Union all only combines two result sets.

Because of the Efficiency of distinct, the efficiency of Union all is higher than that of union.

Temporary table problems:
You don't need it if you don't need it. You don't find its advantages;
It can be used for debugging.