Before we begin, a quick recap and re-introduction to the tables I'll be using throughout this tutorial seems to be in order. In order to explain subqueries, I'll be using a fictitious company's accounting database, which consists of the following four tables:

1. The "services" table: The fictitious system under discussion consists of a service company's accounting database. This company offers customers a number of outsourced services, each of which is associated with a fee and has a unique service ID. This information is stored in a "services" table, which looks like this:

  mysql> SELECT * FROM services;

  +-----+------------------+---------+

  | sid | sname | sfee |

  +-----+------------------+---------+

  | 1 | Accounting | 1500.00 |

  | 2 | Recruitment | 500.00 |

  | 3 | Data Management | 300.00 |

  | 4 | Administration | 500.00 |

  | 5 | Customer Support | 2500.00 |

  | 6 | Security | 600.00 |

  +-----+------------------+---------+

  6 rows in set (0.16 sec)

  

2. The "clients" table: The company also has a list of its current clients stored in a separate "clients" table. Each client is identified with a unique customer ID.

  mysql> SELECT * FROM clients;

  +-----+-----------------------------+

  | cid | cname |

  +-----+-----------------------------+

  | 101 | JV Real Estate |

  | 102 | ABC Talent Agency |

  | 103 | DMW Trading |

  | 104 | Rabbit Foods Inc |

  | 110 | Sharp Eyes Detective Agency |

  +-----+-----------------------------+

  5 rows in set (0.00 sec)

  

3. The "branches" table: Each customer may have one or more branch offices. The "branches" table lists the branch offices per customer, together with each branch's location. Each branch has a description, a unique branch ID, and a foreign key reference to the customer ID.

  mysql> SELECT * FROM branches;

  +------+-----+--------------------------------+------+

  | bid | cid | bdesc | bloc |

  +------+-----+--------------------------------+------+

  | 1011 | 101 | Corporate HQ | CA |

  | 1012 | 101 | Accounting Department | NY |

  | 1013 | 101 | Customer Grievances Department | KA |

  | 1041 | 104 | Branch Office (East) | MA |

  | 1042 | 104 | Branch Office (West) | CA |

  | 1101 | 110 | Head Office | CA |

  | 1031 | 103 | N Region HO | ME |

  | 1032 | 103 | NE Region HO | CT |

  | 1033 | 103 | NW Region HO | NY |

  +------+-----+--------------------------------+------+

  9 rows in set (0.01 sec)

  

4. The "branches_services" table: Services supplied to each branch office are listed in this table, which contains pairs of branch IDs and service IDs (foreign keys into the "branches" and "services" table respectively).

  mysql> SELECT * FROM branches_services;

  +------+-----+

  | bid | sid |

  +------+-----+

  | 1011 | 1 |

  | 1011 | 2 |

  | 1011 | 3 |

  | 1011 | 4 |

  | 1012 | 1 |

  | 1013 | 5 |

  | 1041 | 1 |

  | 1041 | 4 |

  | 1042 | 1 |

  | 1042 | 4 |

  | 1101 | 1 |

  | 1031 | 2 |

  | 1031 | 3 |

  | 1031 | 4 |

  | 1032 | 3 |

  | 1033 | 4 |

  +------+-----+

  16 rows in set (0.11 sec)

  

In order to use the examples in this tutorial, you'll need to recreate these tables in your MySQL 4.1 database server. Instructions and SQL code for doing so are available in the first part of this article. Once you're done reading that, come back and let's continue.