A SELECT statement can reference data in two or more tables and the relational join
combines the data from the referenced tables.
In this way, the SELECT statement defines a join of specified tables to retrieve data
more efficiently than without defining a join of tables.
You can specify both inner joins and outer joins:
An inner join selects data from two or more tables that meet specific join conditions.
Each source must be named and the join condition, that is the common relationship
among the tables to be joined, can be on an ON clause or a WHERE clause.
The outer join is an extension of the inner join that includes rows that qualify for
a simple inner join, as well as a specified set of rows that do not match the join
conditions expressed by the query.