Question

I have simple app where are two tables: Enemies and Weapon. Every Weapon must belong to some Enemy. Therefore Weapon has a column ReferenceID that references EnenyID.
I can easily check how many enemies do have a weapon:

SELECT COUNT(*) FROM Weapons JOIN Enemies ON Weapons.RefID=Enemies.ID

But how can I find out vice versa how many enemies do not have any weapon associated? I.e., how many Enemies here are whose ReferenceID is not used by any Weapon?

Answer 1

SELECT COUNT(*)
FROM Enemies
WHERE NOT EXISTS (SELECT * FROM Weapons WHERE Weapons.RedID=Enemies.ID)