To find and delete duplicate rows from a table, you can execute the query below. Note that this is for Oracle. You can use it for other databases but you need to change the ‘rowid’ then.
delete from table_name A where a.rowid > any ( select B.rowid from table_name B where A.column_name = B.column_name );
You need to change the ‘table_name’ by your table name an the ‘column_name’ by your column you want to check on.
To explain it simply, I will show an example.
Take, for example, a table named persons. In that table, there are the following fields: id, username, first_name, last_name, emailaddress
Let’s say we want to check that username and emailaddress are unique, we need to use the following query:
delete from persons A where a.rowid > any ( select B.rowid from persons B where A.username = B.username and A.emailaddress = B.emailaddress );
Don’t replace rowid by id. In oracle rowid will always be 1,2,3,4… If you want to use it for another sql language, search google for rowid alternatives.
- Person[id=123, username=laurent, first_name=laurent, last_name=hinoul, email@example.com]
- Person[id=456, username=laurent, first_name=test, last_name=test, firstname.lastname@example.org]
The first one will be kept and the last one will be removed because username and emailaddress are equal and the first rowid is less than the second one.