Question

I’m having some troubles with a MySQL Query.

I want to select all the elements from a table, grouping from a certain column, BUT only retrieve those where ALL the elements in the group matches the WHERE clause.

For example, I have a table called “pets” with 3 columns:

Id| animal | name
1 | dog    | marge
2 | dog    | homer
3 | cat    | marge
4 | cat    | lenny
5 | rabbit | homer
5 | rabbit | carl
6 | rabbit | marge

And I want to pick all the animals where ALL his group members have name IN(‘homer’,’bart’,’marge’,’lisa’,’maggie’)

If you can’t understand my question, let me know. Thanks!

Answer 1

Put another way, it’s ‘Select all the animals for whom the number of corresponding names not matching the condition IN ('homer','bart','marge','lisa','maggie') is 0’.

So it could be implemented also like this:

SELECT animal
FROM pets
GROUP BY animal
HAVING COUNT(name NOT IN ('homer','bart','marge','lisa','maggie') OR NULL) = 0
  /* or: SUM(name NOT IN ('homer','bart','marge','lisa','maggie')) = 0 */

This expression

name NOT IN ('homer','bart','marge','lisa','maggie') OR NULL

results in either 1 or NULL, consequently causing COUNT either to count the corresponding occurrence of name or to omit it. (As to why OR NULL is there and how the entire expression works, I’ll refer you to this question: Why do I need "OR NULL" in MySQL when counting rows with a condition.)

As you can see, you could alternatively use SUM to do the same, and the expression for SUM is apparently shorter, as the OR NULL part isn’t needed there. I usually prefer COUNT() to SUM() when I need to count things, as opposed to adding up arbitrary values. But, to my knowledge, there’s no advantage of one before the other, so which function to use is up to your personal preferences/tastes.