Mysql & IFNULL, NULLIF, ISNULL_MySQL

I recently used some functions in mysql, some of which were first encountered, so I recorded that: 1 isnull (expr) usage: expr is null, then isnull () returns 20540; 1, otherwise, 20540 is returned; 0 is returned. 2 IFNULL (expr1, expr2) usage: If you recently used some functions in mysql, some of them are first encountered, so record them:

1. isnull (expr)

Usage: if expr is null, the return value of isnull () is 1; otherwise, the return value is 0.
2. IFNULL (expr1, expr2)

Usage: if expr1 is not NULL, the returned value of IFNULL () is expr1; otherwise, the returned value is expr2.

3. NULLIF (expr1, expr2)

Usage: if expr1 = expr2 is true, the return value is NULL. Otherwise, the return value is expr1.

This is the same as case when expr1 = expr2THEN null else expr1 END.