如何使用 LEFT JOIN 从 MySQL 表中去除重复值？

首先，让我们创建一个表 –

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 命令将一些记录插入表中 –

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)

再次检查表中的记录。

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)

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