Summary

The multitable query is the bread and butter of every RDBMS operation. It combines data in a variety of ways into a single resultset, sometimes incorporating convoluted business logic.

Joining tables is one of the most important concepts to learn in the entire SQL world, and should not be taken lightly.

There are several different types of joins, which could be divided into two broad categories — inner join and outer join. The conditions on which tables could be joined fall into either equijoin or nonequijoin, depending on how columns that are used to join the tables are compared (equal, greater, less than, etc.).

The tables are not the only objects that could be joined in the query. Other objects can be views, or even inline views. A rich arsenal of the SQL language — operators, functions, subqueries — can be used to produce a join query resultset.

To improve database performance when running queries involving complex JOIN statements, a system of hints can be employed. The hints are strictly RDBMS implementation-specific, and are not part of the SQL standard.