Finding duplicate values in MySQL

manpreet Best Answer 2 years ago

I have a table with a varchar column, and I would like to find all the records that have duplicate values in this column. What is the best query I can use to find the duplicates?

manpreet 2 years ago

SELECT  *
FROM    mytable mto
WHERE   EXISTS
        (
        SELECT  1
        FROM    mytable mti
        WHERE   mti.varchar_column = mto.varchar_column
        LIMIT 1, 1
        )

This query returns complete records, not just distinct varchar_column's.

This query doesn't use COUNT(*). If there are lots of duplicates, COUNT(*) is expensive, and you don't need the whole COUNT(*), you just need to know if there are two rows with same value.

Having an index on varchar_column will, of course, speed up this query greatly.