Mysql & Ifnull, Nullif, ISNULL

Some of the functions recently used in MySQL, some of which are also first encountered, are recorded as follows:

1. IsNull (expr)

usage: expr is null, then the return value of IsNull () is 1, otherwise the return value is 0.
2. Ifnull (EXPR1,EXPR2)

Usage: If EXPR1 is not NULL, the return value of Ifnull () is expr1; Otherwise its return value is EXPR2.

3.nullif (EXPR1,EXPR2)

Usage: if Expr1 =EXPR2 is established, the return value is NULL, otherwise the return value is EXPR1.

this and case when expr1 = Expr2     Then NULL ELSE expr1 End is the same.