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法律基于一或两列选择所有重复的 MySQL 行？
为此，请将子查询与 HAVING 子句结合使用。我们首先创建一个表−
mysql> create table DemoTable ( StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY, StudentFirstName varchar(20), StudentLastName varchar(20) ); Query OK, 0 rows affected (0.27 sec)
使用 insert 命令在表中插入一些记录−
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Smith');
Query OK, 1 row affected (0.04 sec)
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('Carol','Taylor');
Query OK, 1 row affected (0.04 sec)
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Doe');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Brown');
Query OK, 1 row affected (0.05 sec)
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('David','Miller');
Query OK, 1 row affected (0.06 sec)使用 select 语句显示表中的所有记录−
mysql> select *from DemoTable;
这将产生以下输出−
+-----------+------------------+-----------------+ | StudentId | StudentFirstName | StudentLastName | +-----------+------------------+-----------------+ | 1 | John | Smith | | 2 | Carol | Taylor | | 3 | John | Doe | | 4 | John | Brown | | 5 | David | Miller | +-----------+------------------+-----------------+ 5 rows in set (0.00 sec)
以下是基于一或两列选择所有重复行的查询。在此，我们计算出现次数超过一次的名字，即重复项−
mysql> select StudentId from DemoTable where StudentFirstName=(select StudentFirstName from DemoTable having count(StudentFirstName) > 1);
这将产生以下输出−
+-----------+ | StudentId | +-----------+ | 1 | | 3 | | 4 | +-----------+ 3 rows in set (0.03 sec)
更新时间：30-Jul-2019
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