How to Effectively Test Inequality of Nullable Columns in SQL?

Understanding the Null Inequality

In the realm of SQL, the equality of NULL values poses a unique challenge. The consensus suggests using the expression "where ((A=B) OR (A IS NULL AND B IS NULL))" to test equality between nullable columns. However, this approach raises a question when testing inequalities.

According to ternary logic, which considers NULL as an unknown value, the inequality expression "where (A<>B)" is sufficient to evaluate the inequality of nullable columns. This logic deems that the equality of unknown values is unknown, hence the result is not TRUE.

For example, consider the following scenarios:

1. If both A and B are NULL, the expression "where (A<>B)" returns FALSE according to ternary logic, indicating that their inequality is unknown.
2. If either A or B is NULL, the expression again returns FALSE, indicating that their inequality cannot be determined.

Additional Conditions

The expression "WHERE ((A <> B) OR (A IS NOT NULL AND B IS NULL) OR (A IS NULL AND B IS NOT NULL))" is an alternative approach that explicitly checks for all possible combinations of NULL and non-NULL values. However, it is unnecessary in situations where the goal is to evaluate the inequality of nullable columns.

In conclusion, for nullable columns, the expression "where (A<>B)" is sufficient to evaluate their inequality, as NULL is considered an unknown value in ternary logic.

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