12.1 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.
|
