Minimal set of attributes

Less likely to have its values changed

Less likely to lose uniqueness in the future

With fewest characters

Easier to use from the user's point of view

Select tuples in a relation that satisfy selection condition

Select certain attributes from the relation and discards other attributes

Result includes tuples in 2 relations and duplicates are removed

Result must include tuples in 2 relations

Result includes tuples in one relation only

Combine tuples with same value in join condition

Combine tuples and keep all tuples even if there is no matching tuple

Combine tuples and keep all tuples if there is matching tuples