Why Does Inequality with NULL Values Work Logically in Databases?

Why Null Inequality Behaves Logically with Ternary Logic

In database management systems, the concept of nullity is crucial when comparing values. The conventional approach dictates that to test equality between nullable fields, one must use an OR condition between equality and IS NULL checks. However, when it comes to inequality, it's surprising to find that a simple inequality operator suffices.

The puzzling behavior of inequalities involving null stems from the underlying ternary logic that applies to unknown values. In this context, null represents an unknown state, and comparing two unknown values against each other yields an unknown result.

Consider the scenario:

A = null
B = null

If we ask "Is A not equal to B?", ternary logic dictates that the answer should be unknown, as we cannot determine if the two unknowns are indeed different. This is precisely why the inequality operator (A<>B) returns FALSE.

In contrast, checking equality between A and B with (A=B) would also return FALSE, as two unknown values cannot be considered equal. This behavior highlights the consistent application of ternary logic in database comparisons.

Therefore, when either value being compared can be null, a simple inequality operator is sufficient to test for inequality. The explicit check for nullity with IS NOT NULL is not necessary because inequality accounts for the unknown state of nullable values and returns FALSE accordingly.

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