Foundations of Semantic Web Technologies

Query Languages 291

• R := Distinct(R), if the query contains DISTINCT.

• R := Slice(R, o, l), if the query contains the directives “OFFSET o” and

“LIMIT l.” If o is not given, o is assumed to be 0. If l is not given, it is

replaced by the number of solutions of R less o.

These translations are relatively simple, and the function of each operator is

mostly obvious: OrderBy orders the solution sequence according to the given

sorting conditions, Distinct eliminates multiple occurrences of any solution in

the result, and Slice combines OFFSET and LIMIT to cut down the solution

sequence to the corresponding segment. The operator Project restricts the

domains of all solutions in a result to the variables chosen in SELECT, i.e. each

solution is replaced