MySQL function (collation)

1, Ifnull (), ISNULL (), and Nullif () functions

When querying, the fields are often null, and the above three functions are sometimes used.

　　Ifnull (expr1, EXPR2) Usage:

If the value of EXPR1 is not NULL, the function is returned as EXPR1, otherwise the return value of the function is EXPR2. The return value of the ifnull () function is either a number or a string, depending on the context used.

　　ISNULL (expr) Usage:

If the value of expr is null, then the return value of the function is 1, otherwise the return value is 0.

　　Nullif (expr1, EXPR2) Usage:

if Expr1 = = EXPR2 is established, then the function return value is NULL, otherwise the return value is EXPR1.

As: case when expr1 = Expr2 then null else EXPR1 end;

MySQL function (collation)