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How to remove duplicate values from a MySQL table using LEFT JOIN?
Let us first create a table −
mysql> create table DemoTable ( Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, FirstName varchar(100) ); Query OK, 0 rows affected (0.46 sec)
Insert some records in the table using insert command −
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)Display all records from the table using select statement −
mysql> select *from DemoTable;
This will produce the following output −
+----+-----------+ | Id | FirstName | +----+-----------+ | 1 | Chris | | 2 | Robert | | 3 | Robert | | 4 | John | | 5 | John | | 6 | Mike | +----+-----------+ 6 rows in set (0.00 sec)
Following is the query to remove duplicate values from MySQL table −
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)
Let us check table records once again.
mysql> select *from DemoTable;
This will produce the following output −
+----+-----------+ | Id | FirstName | +----+-----------+ | 1 | Chris | | 2 | Robert | | 4 | John | | 6 | Mike | +----+-----------+ 4 rows in set (0.00 sec)
Updated on: 24-Sep-2019
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