IFNULL
The IFNULL() function return the first expression if it is not NULL. Otherwise, the second expression is returned.
Syntax
IFNULL( <expr1>, <expr2>)
Arguments
| Arguments | Description | 
|---|---|
| <expr1> | Any general expression of any data type. | 
| <expr2> | Any general expression that evaluates to the same data type as \<expr1>. | 
Return Type
The data type of the returned value is the data type of \<expr1>.
Examples
SELECT a, b, IFNULL(a, b) FROM t;
+------+------+--------------+
| a    | b    | IFNULL(a, b) |
+------+------+--------------+
|    0 |    1 |            0 |
|    0 | NULL |            0 |
| NULL |    1 |            1 |
| NULL | NULL |         NULL |
+------+------+--------------+