mysql中的NULL的判断

mysql>deletefrom`zhangchao`wherex='NULL';

QueryOK,0rowsaffected(0.00sec)

mysql>deletefrom`zhangchao`wherex=NULL;
QueryOK,0rowsaffected(0.00sec)

mysql>select*fromzhangchao;
+------+-------+
|x|y|
+------+-------+
|xxx|yyy|
|NULL|NULL|
|xxx|james|
|xxx|lucy|
|xxx|alex|
+------+-------+
5rowsinset(0.00sec)

mysql>deletefrom`zhangchao`wherexisnull;
QueryOK,1rowaffected(0.00sec)

mysql>select*fromzhangchao;
+------+-------+
|x|y|
+------+-------+
|xxx|yyy|
|xxx|james|
|xxx|lucy|
|xxx|alex|
+------+-------+
4rowsinset(0.00sec)

mysql>