SQL and Relational Algebra

Relational Algebra operators:

• Selection corresponds to WHERE clause in SQL syntax
• Projection corresponds to SELECT clause in SQL syntax
• Cross-product combines fields of selected tables
• Join is basically a cross-product followed by a selection followed by a projection
• conditional joins: cross-product followed by a selection
• equijoins: cross-product followed by a 'equivalence' selection followed by a projection to exclude repeated fields in the resulting table
• natural join: equijoin on all fields that have the same name

http://en.wikipedia.org/wiki/Relational_algebra