For example, if it were the case that NULL = NULL, really counterintuitive stuff would happen on joins because a null cell would match with every other null ce

For example, if it were the case that NULL = NULL, really counterintuitive stuff would happen on joins because a null cell would match with every other null cell you are joining on: person name home_address --------------------------- "Alice" NULL "Bob" "123 Jump Street" letter return_address description ---------------------------------- NULL "Ransom Letter" NULL "Spy Document" "123 Jump Street" "Hello, from Bob" Then SELECT name, description FROM person INNER JOIN letter ON home_address = return_address would return name description ------------ "Alice" "Ransom Letter" "Alice" "Anonymous Spy Document" "Bob" "Hello from Bob" So now Alice is associated with a bunch of letters she didn't necessarily write because she doesn't have a home address.