Learning SQL

Chapter 5. Querying Multiple Tables

Because relational database design mandates that independent entities be placed in separate tables, you will need a mechanism for bringing multiple tables together in the same query. This mechanism is known as a join, and this chapter will concentrate on the simplest and most common join, the inner join; Chapter 10 will demonstrate all of the different join types.

What Is a Join?

Queries against a single table are certainly not rare, but you will find that most of your queries will require two, three, or even more tables. To illustrate, let's look at the definitions for the employee and department tables and then define a query that retrieves data from both tables:

    mysql> DESC employee;
    +--------------------+----------------------+------+-----+------------+
    | Field              | Type                 | Null | Key | Default    |
    +--------------------+----------------------+------+-----+------------+
    | emp_id             | smallint(5) unsigned |      | PRI | NULL       |
    | fname              | varchar(20)          |      |     |            |
    | lname              | varchar(20)          |      |     |            |
    | start_date         | date                 |      |     | 0000-00-00 |
    | end_date           | date                 | YES  |     | NULL       |
    | superior_emp_id    | smallint(5) unsigned | YES  | MUL | NULL       |
    | dept_id            | smallint(5) unsigned | YES  | MUL | NULL       |
    | title              | varchar(20)          | YES  |     | NULL       |
    | assigned_branch_id | smallint(5) unsigned | YES  | MUL | NULL       |
    +--------------------+----------------------+------+-----+------------+
    9 rows in set (0.11 sec)

    mysql> DESC department; +---------+----------------------+------+-----+---------+ | Field ...