3. Understanding SQL Data Types : Nulls : Nulls and Comparisons
 
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Nulls and Comparisons
Because a null is not a value, it cannot be compared to any other value (including another null value). For example, the following WHERE clause evaluates to false if one or both of the columns is null:
WHERE columna = columnb
Similarly, the WHERE clause:
WHERE columna < 10 OR columna >= 10
is true for all numeric values of columna, but false if columna is null.