select * from table where ifnull(fieldName,’’) = ‘’

测试用例：

create table t (a int,b varchar(2));
insert t(a) values (1);
insert t(a,b) values (2,'');
insert t(a,b) values (3,'c');
select * from t where b is null or b = '';
select * from t where ifnull(b, '') = '';
+------+------+
| a    | b    |
+------+------+
|    1 | NULL |
|    2 |      |
+------+------+
2 rows in set (0.00 sec)