2 different small query vs 1 query with subquery

Question

I have table like this

name       | personal_number 
-----------------------------------------
Jon        | 222
Alex       | 555
Jon        | 222
Jimmy               


        

Answer 1

This should be quicker:

SELECT  name  FROM mytable join (
        SELECT  personal_number  FROM mytable  GROUP BY personal_number
        HAVING COUNT(*) > 1
)a using (personel_number)

Edit: If this is faster than variant 1, then it means at variant 1 mysql reproduces the inner table for each record again and again.