总结:
mysql中:
1.isnull(exper) 判断exper是否为空,是则返回1,否则返回0
2.ifnull(exper1,exper2)判断exper1是否为空,是则用exper2代替
3.nullif(exper1,exper2)如果expr1= expr2 成立,那么返回值为NULL,否则返回值为 expr1。
mysql> SELECT IFNULL(1,0);
-> 1
mysql> SELECT IFNULL(NULL,10);
-> 10
mysql> SELECT IFNULL(1/0,10);
-> 10
mysql> select isnull(1+1);
-> 0
mysql> select isnull(1/0);
-> 1
mysql> SELECT NULLIF(1,1);
-> NULL
mysql> SELECT NULLIF(1,2);
-> 1
源文:https://blog.csdn.net/lch_2016/article/details/81007761