你这个问题是否可以理解为“验证表里的某一不确定的字段的值，没有重复”？
如果是的话，那么你就可以先查出表的总记录数，然后在查出该字段去重之后的总记录数，
对比两次结果，如果数量相同则没有重复；如果数量不相等，则代表有重复的内容。
下面是一个示例：
mysql> select * from user;
+----+------+-----------+
| id | name | address |
+----+------+-----------+
| 1 | aaa | beijing |
| 2 | bbb | shanghai |
| 3 | ccc | hangzhou |
| 5 | aaa | chongqing |
+----+------+-----------+
4 rows in set (0.00 sec)
mysql> select (select count(*) from user) = (select count(*) from (select distinct name from user) t);
+-----------------------------------------------------------------------------------------+
| (select count(*) from user) = (select count(*) from (select distinct name from user) t) |
+-----------------------------------------------------------------------------------------+
| 0 |
+-----------------------------------------------------------------------------------------+
1 row in set (0.01 sec)
mysql> delete from user where id=5;
Query OK, 1 row affected (0.05 sec)
mysql> select * from user;
+----+------+----------+
| id | name | address |
+----+------+----------+
| 1 | aaa | beijing |
| 2 | bbb | shanghai |
| 3 | ccc | hangzhou |
+----+------+----------+
3 rows in set (0.00 sec)
mysql> select (select count(*) from user) = (select count(*) from (select distinct name from user) t);
+-----------------------------------------------------------------------------------------+
| (select count(*) from user) = (select count(*) from (select distinct name from user) t) |
+-----------------------------------------------------------------------------------------+
| 1 |
+-----------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
