Question

I have a table in a MySQL database with an ID column. This is not a key of the table and several rows can have the same ID.

I don’t really know SQL but I already figured out how to obtain the number of distinct IDs:

SELECT COUNT(DISTINCT ID) FROM mytable;

Now I want to count only those IDs which appear more than 2 times in the table.

So if the ID column contains the values

 3 4 4 5 5 5 6 7 7 7

the query should return 2.

I have no idea how to do this. I hope someone can help me!

Btw, my table contains a huge number of rows. So if there are several possibilities I would also be happy to know which solution is the most efficient.

Answer 1

Try this:

SELECT COUNT(ID) FROM (
    SELECT ID FROM mytable
    GROUP BY ID
    HAVING COUNT(ID) > 2) p