Question

I’m having a problem grokking this code

SELECT * FROM sometable WHERE EXISTS (
   SELECT 1 FROM sometable
   GROUP BY relevant_field
   HAVING count(*) > 1) 

If I use a JOIN, I specify the link explicitly, but here I don’t so I don’t have clear concept of what’s happening inside MySQL.

How does MySQL know how to link the rows from the inner sometable to the outer sometable?

Answer 1

Your query as it is will either returns all rows from sometable or no rows at all, depending on whether the parenthesized SELECT will return rows or not.

In other words, MySQL does not know how to and will not link the rows from the inner to the outer sometable in this particular query.

The equivalent to a join would be:

SELECT * FROM sometable a WHERE EXISTS (
   SELECT 1 FROM sometable b
   WHERE a.relevant_field = b.relevant_field
   GROUP BY b.relevant_field
   HAVING count(*) > 1)