NULL Values

If the target of a reference is not in the persistent store, then the target is NULL. For conditional expressions containing NULL, the query language uses the semantics defined by SQL92. Briefly, these semantics are as follows:

• If a comparison or arithmetic operation has an unknown value, it yields a NULL value.

• Two NULL values are not equal. Comparing two NULL values yields an unknown value.

• The IS NULL test converts a NULL persistent field or a single-valued relationship field to TRUE. The IS NOT NULL test converts them to FALSE.

• Boolean operators and conditional tests use the three-valued logic defined by Table 21–7 and Table 21–8. (In these tables, T stands for TRUE, F for FALSE, and U for unknown.)

Table 21–7 AND Operator Logic

AND	T	F	U
T	T	F	U
F	F	F	F
U	U	F	U

Table 21–8 OR Operator Logic

OR	T	F	U
T	T	T	T
F	T	F	U
U	T	U	U