Indique los códigos de los proveedores que venden a un único proyecto.

Select distinct S#
From SPJ X
Where S# not in (select S#
From S
Where X.J#<>J#);

Algebra: SPJ[S#] minus
((SPJ[S#, J#] Join ((SPJ rename J# as Jnum)(S#, Jnum)) where J#<> Jnum) [S#])