Question

This seems so basic, I’m flabbergasted for lack of a better word. I have two tables, let’s call them albums and artists

CREATE TABLE `albums` (
  `album_id` bigint(20) NOT NULL AUTO_INCREMENT,
  `artist_id` bigint(20) DEFAULT NULL,
  `name` varchar(200) NOT NULL,
  PRIMARY KEY (`album_id`)
)
CREATE TABLE `artists` (
  `artist_id` bigint(20) NOT NULL AUTO_INCREMENT,
  `name` varchar(250) NOT NULL,
  PRIMARY KEY (`artist_id`)
)

There are a few hundred thousand reconds in each table. Some of the album rows have a null artist_id, this is expected.

However, when I perform the following query to find artists without albums:

SELECT * FROM artists WHERE artist_id NOT IN (SELECT artist_id FROM albums)

… the query returns zero results. I know that this is not true. So I tried this one:

SELECT * FROM artists WHERE artist_id NOT IN (SELECT artist_id FROM albums WHERE artist_id IS NOT NULL)

… and I get back a couple thousand rows. My question is: Why did the first query seem to operate on the idea that any number = NULL? Or is this an odd effect that NULL has on the IN() statement? I feel like this is something basic that I’ve missed. I don’t usually use NULL in my db tables at all.

Answer 1

This is why NOT EXISTS is semantically correct

SELECT * FROM artists ar
WHERE  NOT EXISTS
   (SELECT * FROM albums al WHERE ar.artist_id = al.artist_id)

Logic:

• NOT IN (x, y, NULL) is actually
  • NOT (x OR y OR NULL) is actually
    • (NOT x) AND (NOT y) AND (NOT NULL)

So NULL invalidates the whole NOT IN