In this post we look at ways to remove all duplicate rows except one in an SQL database. For all examples in this article, we'll be using the following MySQL "user" table: +---+---+ | id | name | +
12.2012/ Category:How To/ Tags:sql help Today somebody asked me how to remove duplicates which accidentally made it into a table. The problem is: A normal DELETE won't do, because you would delete both values - not just the one which is in there twice. ...
To delete similar records, i.e. where the records are not the same but one field is the same and only one copy needs to be preserved, try the following SQL: delete T1 from MyTable T1, MyTable T2 where T1.dupField = T2.dupField ...
This returns you a single row for each combination. This includes the rows without duplicates. To return just the copied values you need to filter the results. This is those where the count is greater than one. You can do this with a having clause, like so: Copy code snippet Copied to ...
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If we look at the result set, we can easily understand thatBookNumber: 3andBookNumber: 4are duplicate rows. Previously, it was explained in detailhow to remove duplicates in SQL. We must delete them to keep the database consistent.
SELECT id,name,age, ROW_NUMBER() OVER (PARTITION BY id, name, age ORDER BY id) AS Duplicates FROM Test ) DELETE FROM CTE WHERE Duplicates > 1 GO Now check the table to make sure duplicates are being removed from table. Related:
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WHERE [row-number-column] > 1;Copy The output below shows the two duplicate entries in the example table. Remove every entry except the ones marked with1to delete duplicate rows. Use theDELETEquery with theROW_NUMBER()function as the filter: ...
2. Flagging Duplicates With row_number() Function While thegroup byandhavingcombination is the simplest way to find and flag duplicates within a table, there is an alternate way to find duplicates using therow_number()function. Therow_number()function is a part of theSQL window functionscatego...