Question

Lets say I have a column named source in a table x. Individual entries can be like;

Id   c1   c2      source ...
1    a     b      something
2    b     a      something  
3    a     b      somethingelse
4    c     a      somethingelse
5    a     b      something
6    b     c      something

How can I delete entries with less than 3 same elements in source? For example since source value somethingelse appears 2 times, I need all entries that have somethingelse removed.

Answer 1

DELETE  a
FROM    tableName a
        INNER JOIN
        (
            SELECT  source
            FROM    tableName
            GROUP BY SOURCE
            HAVING COUNT(*) < 3
        ) b ON a.source = b.source

• SQLFiddle Demo

One more thing to do for faster performance, add an INDEX to column SOURCE.