Learning SQL, 2nd Edition

Chapter 3. Query Primer

So far, you have seen a few examples of database queries (a.k.a. select statements) sprinkled throughout the first two chapters. Now it’s time to take a closer look at the different parts of the select statement and how they interact.

Query Mechanics

Before dissecting the select statement, it might be interesting to look at how queries are executed by the MySQL server (or, for that matter, any database server). If you are using the mysql command-line tool (which I assume you are), then you have already logged in to the MySQL server by providing your username and password (and possibly a hostname if the MySQL server is running on a different computer). Once the server has verified that your username and password are correct, a database connection is generated for you to use. This connection is held by the application that requested it (which, in this case, is the mysql tool) until the application releases the connection (i.e., as a result of your typing quit) or the server closes the connection (i.e., when the server is shut down). Each connection to the MySQL server is assigned an identifier, which is shown to you when you first log in:

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 11
Server version: 6.0.3-alpha-community MySQL Community Server (GPL)

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

In this case, my connection ID is 11. This information might be useful to your database administrator if something ...