如何使用 LEFT JOIN 从 MySQL 表中删除重复值?
mysqlmysqli database
首先我们创建一个表 −
mysql> create table DemoTable ( Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, FirstName varchar(100) ); Query OK, 0 rows affected (0.46 sec)
使用 insert 命令在表中插入一些记录 −
mysql> insert into DemoTable(FirstName) values('Chris');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable(FirstName) values('Robert');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable(FirstName) values('Robert');
Query OK, 1 row affected (0.07 sec)
mysql> insert into DemoTable(FirstName) values('John');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable(FirstName) values('John');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable(FirstName) values('Mike');
Query OK, 1 row affected (0.09 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 −
+----+-----------+ | Id | FirstName | +----+-----------+ | 1 | Chris | | 2 | Robert | | 3 | Robert | | 4 | John | | 5 | John | | 6 | Mike | +----+-----------+ 6 rows in set (0.00 sec)
以下是从 MySQL 表中删除重复值的查询 −
mysql> delete tbl from DemoTable tbl left join( select min(Id) as Id, FirstName from DemoTable group by FirstName ) tbl1 ON tbl.Id = tbl1.Id AND tbl.FirstName = tbl1.FirstName where tbl1.Id IS NULL; Query OK, 2 rows affected (0.16 sec)
让我们再次检查表记录 −
mysql> select *from DemoTable;
这将产生以下输出 −
+----+-----------+ | Id | FirstName | +----+-----------+ | 1 | Chris | | 2 | Robert | | 4 | John | | 6 | Mike | +----+-----------+ 4 rows in set (0.00 sec)
