Introduction

For the most part, recipes in earlier chapters have used single tables. But for any application of even moderate complexity, it's likely that you'll need to use multiple tables. Some queries simply cannot be answered using a single table, and the real power of a relational database comes into play when you start to relate the information in tables to each other. There are several reasons to use multiple tables:

• To combine records from tables to obtain more comprehensive information than can be obtained from individual tables alone
• To hold intermediate results for a multiple-stage operation
• To insert, delete, or update records in one table based on information in another

When you use multiple tables, they may come from the same database or from different databases. On occasion, you may even need to use tables that come from databases hosted by different MySQL servers. For the first two cases, you'll need to know how to refer to columns from the different tables, which may involve using table aliases or qualifying table names with a database name. In the third case, you'll need to open a connection to each server and relate the information from them yourself.