If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.
mysql>mysql> SELECT IFNULL(1,0);+-------------+| IFNULL(1,0) |+-------------+| 1 |+-------------+1 row in set (0.00 sec)mysql>mysql> SELECT IFNULL(NULL,10);+-----------------+| IFNULL(NULL,10) |+-----------------+| 10 |+-----------------+1 row in set (0.00 sec)mysql>mysql> SELECT IFNULL(1/0,10);+----------------+| IFNULL(1/0,10) |+----------------+| 10.0000 |+----------------+1 row in set (0.00 sec)mysql>mysql> SELECT IFNULL(1/0,'yes');+-------------------+| IFNULL(1/0,'yes') |+-------------------+| yes |+-------------------+1 row in set (0.00 sec)mysql>
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