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 semanticsselect 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 returnedselect 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 returnedselect 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 returnedselect 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 returnedselect 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 AlsoEJB query language
Using EJB query