Null value semantics

The following describe the semantics of NULL values...

• Comparison or arithmetic operations with an unknown (NULL) value yield an unknown value

• Path expressions that contain NULL evaluate to NULL

• The IS NULL and IS NOT NULL operators can be applied to path expressions and return TRUE or FALSE. Boolean operators AND, OR and NOT use three valued logic.

AND	True	False	Unknown
True	True	False	Unknown
False	False	False	False
Unknown	Unknown	False	Unknown

OR	True	False	Unknown
True	True	True	True
False	True	False	Unknown
Unknown	True	Unknown	Unknown

	NOT
True	False
False	True
Unknown	Unknown

 

Example: Null value semantics

select object(e) from EmpBean where e.salary > 10  and e.dept.budget > 100

If salary is NULL the evaluation of e.salary > 10 returns unknown and the employee object is not returned. If the cmr field dept or budget is NULL evalution of e.dept.budget > 100 returns unknown and the employee object is not returned

select object(e) from EmpBean where e.dept.budget is null

If dept or budget is NULL evaluation of e.dept.budget is null returns TRUE and the employee object is returned

select object(e) from EmpBean e ,  in (e.dept.emps) e1 where e1.salary > 10

If dept is NULL, then the multivalued path expression e.dept.emps results in an empty collection (not a collection that contains a NULL value). An employee with a null dept value will not be returned

select object(e) from EmpBean e where  e.dept.emps is empty

If dept is NULL the evaluation of the predicate in unknown and the employee object is not returned

select object(e) from EmpBean e , EmpBean e1  where e member of  e1.dept.emps

If dept is NULL evaluation of the member of predicate returns unknown and the employee is not returned.

 

See Also

EJB query language
WHERE clause
Expressions
Using EJB query

 

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