What does the JOIN keyword do in SQL?
A join combines records from two or more tables in a relational database. In the Structured Query Language (SQL), there are three types of joins: inner, outer, and cross. Outer joins are subdivided further into left outer joins, right outer joins, and full outer joins.
Mathematically, join is relation composition, the fundamental operation in relational algebra, and generalizing function composition.
see: http://en.wikipedia.org/wiki/JOIN
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