The difference between two relations produces a third relation containing all of the rows in the 1st relation that are not in the 2nd relation. The symbol, -, indicates difference. The projection on a relation over a list of specified attributes produces a relation whose columns are just those attributes in the order specified, and with duplicate rows eliminated.
Selection is denoted by a semicolon.
Notice that the relation resulting from a selection has the same structure as the original relation. The join of two relations produces a third relation containing all of the attributes from each of the two relations. A row is created in the new relation whenever the two original relations have equal values for any common attributes. The join relation allows us to relate any two relations in a database which have one or more common attributes.