MySQL - SELECT WHERE field IN (subquery) - Extremely slow why?

Question

I\'ve got a couple of duplicates in a database that I want to inspect, so what I did to see which are duplicates, I did this:

SELECT relevant_field
FROM some         


        

Answer 1

SELECT st1.*
FROM some_table st1
inner join 
(
    SELECT relevant_field
    FROM some_table
    GROUP BY relevant_field
    HAVING COUNT(*) > 1
)st2 on st2.relevant_field = st1.relevant_field;

I've tried your query on one of my databases, and also tried it rewritten as a join to a sub-query.

This worked a lot faster, try it!