Relational Operators

There are 8 relational operators:

1. SELECT Selects rows from a table that have specified attribute values.

2. PROJECT Selects rows made up of a subset of columns from a table.

3. JOIN Associates entries from two tables on the basis of matching column values.
4. PRODUCT Builds a relation from two specified relations consisting of all possible combinations of rows, one from each of the two relations.

5. UNION Builds a relation consisting of all rows appearing in either or both of the two relations.

6. INTERSECT Builds a relation consisting of all rows appearing in both of the two relations.

7. DIFFERENCE Builds a relation consisting of all rows appearing in the first and not in the second of the two relations.

8. DIVIDE Takes two relations, one binary and one unary, and builds a relation consisting of all values of one column of the binary relation that match, in the other column, all values in the unary relation. Ex. A: B: a x x

   a y y

   a z

   b x

   c y

   => A DIVIDE B: a Basically, A/B = <m> such that for every n value in a tuple of B there is <m, n> in A.