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.
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 ...