•A bag = a set with repeated elements
•All operations need to be defined carefully on bags
•{a,b,b,c}È{a,b,b,b,e,f,f}={a,a,b,b,b,b,b,c,e,f,f}
•{a,b,b,b,c,c} – {b,c,c,c,d} = {a,b,b,d}
• sC(R): preserve the number of occurrences
• PA(R): no duplicate elimination
•Cartesian product, join: no duplicate elimination
•Important ! Relational Engines work on bags, not sets !