Up: Handling NULL values
When returning NULL's as field values, the interface returns NULL/1
function with a unique integer argument serving as a skolem constant.
Notice that the above guarantees the expected semantics for the join
statements. In the following example, even if Deptno is NULL for some rows in emp or dept tables, the query still evaluates the join successfully.
| ?- emp(Ename,_,_,_,Deptno),dept(Deptno,Dname,Loc)..