A rule is filter, then project
Take the handout’s first rule: the adults in a person table.
adult(Name) :- person(Name, Age), Age >= 18.The single body atom is the table person.
The comparison Age >= 18 keeps some rows, a filter σ.
The head keeps only the name, column #1, a projection π:
adult := π#1( σ#2 ≥ 18( person ) )
Read it inside out: filter
person down to the adult rows, then project onto the name.
Columns are positional, so Age is #2.