Database Reference
In-Depth Information
Figure 5-20.
Using the IS NOT NULL operator
Joins
Most queries require information from more than one table.
A join
is a relational operation that
produces a table by retrieving data from two (not necessarily distinct) tables and matching their rows
according to a
join specification.
Different types of joins exist, which you'll look at individually, but keep in mind that every join is a
binary
operation; that is, one table is joined to another, which may be the same table since tables can be
joined to themselves. The join operation is a rich and somewhat complex topic. The next sections will
cover the basics.
Inner Joins
An inner join is the most frequently used join. It returns only those rows that satisfy the join
specification. Although in theory any relational operator (such as
>
or
<
) can be used in the join
specification, the equality operator (=) is almost always used. Joins using the equality operator are called
natural joins.
