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>