根据一列或两列选择所有重复的 MySQL 行？
为此，请使用子查询和HAVING子句。让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) );
使用插入命令在表中插入一些记录-
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Smith');
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('Carol','Taylor');
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Doe');
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Brown');
mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('David','Miller');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)
以下是根据一列或两列选择所有重复行的查询。在这里，我们正在计算出现不止一次的名字，即重复-
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)