THREE-VALUED LOGIC

-- this is true

SELECT 1 WHERE 1 = 1

Output : 1

-- this is false

SELECT 1 WHERE 1 = 0

Ouput :No Output

-- this is unknown - it is usually expected to be false, but that only shows 

-- misunderstanding of nulls. It's not false it's only treated as false in the filter

SELECT 1 WHERE 1 = NULL

Ouput :No Output

-- this is also unknown - but logicaly it would seem it would be true

-- but unknown compared to unknown equals uknown and it is treated as false in the filter

SELECT 1 WHERE NULL = NULL

Ouput :No Output