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Deleting duplicate rows from a MySQL database

Esos shows a good way of removing duplicate entries from a database table. Of course, if primary keys were used this shouldn’t happen, but doing a select distinct into another table is neat !

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By Donncha

Donncha Ó Caoimh is a software developer at Automattic and WordPress plugin developer. He posts photos at In Photos and can also be found on Google+ and Twitter.

1 reply on “Deleting duplicate rows from a MySQL database”

Remove duplicate entries. Assume the following table and data.

CREATE TABLE IF NOT EXISTS Test(
pkey int(11) NOT NULL auto_increment,
a int,
b int,
c int,
timeEnter timestamp(14),
PRIMARY KEY (pkey)

);

insert into Test(a,b,c) values (1,2,3),(1,2,3),
(1,5,4),(1,6,4);

mysql> select * from Test;
select * from Test;
+——+——+——+——+———————+
| pkey | a | b | c | timeEnter |
+——+——+——+——+———————+
| 1 | 1 | 2 | 3 | 2004-04-16 10:55:35 |
| 2 | 1 | 2 | 3 | 2004-04-16 10:55:35 |
| 3 | 1 | 5 | 4 | 2004-04-16 10:55:35 |
| 4 | 1 | 6 | 4 | 2004-04-16 10:55:35 |
+——+——+——+——+———————+
4 rows in set (0.00 sec)

mysql>

Note, the first two rows contains duplicates in columns a and b. It contains
other duplicates; but, leaves the other duplicates alone.

mysql> ALTER IGNORE TestdupTest ADD UNIQUE INDEX(a,b);

mysql> select * from Test;
select * from dupTest;
+——+——+——+——+———————+
| pkey | a | b | c | timeEnter |
+——+——+——+——+———————+
| 1 | 1 | 2 | 3 | 2004-04-16 11:11:42 |
| 3 | 1 | 5 | 4 | 2004-04-16 11:11:42 |
| 4 | 1 | 6 | 4 | 2004-04-16 11:11:42 |
+——+——+——+——+———————+
3 rows in set (0.00 sec)

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