Question

I have the following query:

SELECT SUM(count) 
  FROM (SELECT 'artist' AS table_name, COUNT(*) as count FROM artist
        UNION
        SELECT 'persons' AS table_name, COUNT(*) as count FROM persons
        UNION
        SELECT 'track' AS table_name, COUNT(*) as count FROM track)

It works as expected and returns the proper count. But now when I do the following query I get the incorrect count:

SELECT SUM(count) 
  FROM (SELECT COUNT(*) as count FROM artist
        UNION
        SELECT COUNT(*) as count FROM persons
        UNION
        SELECT COUNT(*) as count FROM track)

Why is it the first query gets the proper count and the second one does not?

Answer 1

The UNION eliminates duplicate values, so if two counts happen to be the same, one is eliminated and only one is summed. Try using UNION ALL instead.

SELECT sum(count) FROM
(SELECT COUNT(*) as count FROM artist
UNION ALL
SELECT COUNT(*) as count FROM persons
UNION ALL
SELECT COUNT(*) as count FROM track)