极客教程MySQL 如何检查表中多列中的重复项
要检查MySQL中的重复项，可以使用group by having子句。语法如下。
select yourColumnName1,yourColumnName2,......N,count(*) as anyVariableName from
yourTableName
group by yourColumnName1,yourColumnName2
having count(*) > 1;
为了理解上述语法，让我们创建一个表。创建表的查询如下。
mysql> create table DuplicateDemo
-> (
-> StudentId int not null,
-> StudentFirstName varchar(100),
-> StudentLastName varchar(100),
-> Primary Key(StudentId)
-> );
Query OK, 0 rows affected (0.50 sec)
使用insert命令向表中插入一些记录。查询如下。
mysql> insert into DuplicateDemo values(1,'John','Smith');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DuplicateDemo values(2,'Mike','Jones');
Query OK, 1 row affected (0.28 sec)
mysql> insert into DuplicateDemo values(3,'David','Smith');
Query OK, 1 row affected (0.15 sec)
mysql> insert into DuplicateDemo values(4,'Carol','Taylor');
Query OK, 1 row affected (0.20 sec)
mysql> insert into DuplicateDemo values(5,'David','Smith');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DuplicateDemo values(6,'John','Smith');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DuplicateDemo values(7,'John','Taylor');
Query OK, 1 row affected (0.15 sec)
使用select语句显示表中的所有记录。
查询如下 –
mysql> select *from DuplicateDemo;
以下是输出。
+-----------+------------------+-----------------+
| StudentId | StudentFirstName | StudentLastName |
+-----------+------------------+-----------------+
| 1 | John | Smith |
| 2 | Mike | Jones |
| 3 | David | Smith |
| 4 | Carol | Taylor |
| 5 | David | Smith |
| 6 | John | Smith |
| 7 | John | Taylor |
+-----------+------------------+-----------------+
7 rows in set (0.00 sec)
以下是检查表中重复项的查询。
mysql> select StudentFirstName,StudentLastName,count(*) as Total from DuplicateDemo
-> group by StudentFirstName,StudentLastName
-> having count(*) > 1;
以下是输出。
+------------------+-----------------+-------+
| StudentFirstName | StudentLastName | Total |
+------------------+-----------------+-------+
| John | Smith | 2 |
| David | Smith | 2 |
+------------------+-----------------+-------+
2 rows in set (0.00 sec)
阅读更多：MySQL 教程