%
% Temporal Projection
%
% Haixun Wang, hxwang@cs.ucla.edu
%
% Assume that you have an emp(E#, SAL, D#, Time-Interval)
% relation, sorted by E# and the start time of time 
% intervals, as in all Snodgrass' examples.

%%
database( {
	emp(Emp:string, Sal:integer, Dept:string, Interval:any)
} ).

%%
export empProj1(Emp, Dept, Interval).
export empProj2(Emp, Sal, Interval).
export empProj(Emp, Interval).

%%
empProj(EmpId, coalesce<Interval>)<-emp(EmpId,_,_,Interval).
empProj1(EmpId, DeptId, coalesce<Interval>)<-emp(EmpId,_,DeptId,Interval).
empProj2(EmpId, Sal, coalesce<Interval>)<-emp(EmpId,Sal,_,Interval).

single(coalesce, (Start, End), (Start, End)).
% in multi rule, since Date is sorted, so StartNew>=StartOld
multi(coalesce, (StartNew, EndNew), (StartOld, EndOld), (Start, End))<-
    if (EndOld>=StartNew then Start=StartOld
    else Start=StartNew),
    if (EndNew>=EndOld then End=EndNew
    else End=EndOld).

return(coalesce, (StartNew, EndNew), (StartOld, EndOld), (StartOld, EndOld))<-
    EndOld<StartNew.
return(coalesce, nil, X, X).