Question

So the Table structure is this

          CD id        |          Artist id

I want to return artists that appear more than once in this table, as in with more than one CD.

I’m using Mysql so what is the best SQL query to do this?

Answer 1

First you group your records by the artist_id, then you can use the HAVING clause, which allows you to filter data based on an aggregate, such as COUNT, or SUM

SELECT Artist_id, COUNT(1)
FROM yourTable
GROUP BY Artist_id
HAVING COUNT(1) > 1

Short explanation:
This query will select the artist, as well as a count of the number of times they are in the table (Number of CDs in this instance)

This COUNT is aggregated on the GROUP BY clause, so it will count each grouping of Artist_Id

The HAVING clause then says that we only want to return records that have a COUNT(1) > 1 which will indicate number of CDs for this table.