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. The one exception, count(), is described in the next section.