2.0. 介绍

MySQL Client API Architecture

Each MySQL programming interface covered in this book uses a two-level architecture:

• The upper level provides database-independent methods that implement database access in a portable way that’s the same whether you use MySQL, PostgreSQL, Ora‐ cle, or whatever.
• The lower level consists of a set of drivers, each of which implements the details for a single database system.

2.1. Connecting, Selecting a Database, and Disconnecting

Problem

You need to establish a connection to the database server and shut down the connection when you’re done.

Solution

Each API provides routines for connecting and disconnecting. The connection routines require that you provide parameters specifying the host on which the MySQL server is running and the MySQL account to use. You can also select a default database.

Discussion

Establishing a connection to the MySQL server Every program that uses MySQL does this, no matter which API you use. The details on specifying connection parameters vary between APIs, and some APIs provide more flexibility than others. However, there are many common parameters, such as the host on which the server is running, and the username and password of the MySQL account to use for accessing the server.

Selecting a database Most MySQL programs select a default database.

Disconnecting from the server Each API provides a way to close an open connection. It’s best to do so as soon as you’re done using the server. If your program holds the connection open longer than necessary, the server cannot free up resources allocated to servicing the con‐ nection. It’s also preferable to close the connection explicitly. If a program simply terminates, the MySQL server eventually notices, but an explicit close on the user end enables the server to perform an immediate orderly close on its end.

This section includes example programs that show how to use each API to connect to the server, select the cookbook database, and disconnect. The discussion for each API also indicates how to connect without selecting any default database. This might be the case if you plan to execute a statement that doesn’t require a default database, such as SHOW VARIABLES or SELECT VERSION() . Or perhaps you’re writing a program that enables the user to specify the database after the connection has been made.

2.2. Checking for Errors

Problem

Something went wrong with your program, and you don’t know what.

Solution

Everyone has problems getting programs to work correctly. But if you don’t anticipate problems by checking for errors, the job becomes much more difficult. Add some error- checking code so your programs can help you figure out what went wrong.

Discussion

It’s also a good idea to know how to check for errors and how to retrieve specific error information from the API

When an error occurs, MySQL provides three values:

• A MySQL-specific error number
• A MySQL-specific descriptive text error message
• A five-character SQLSTATE error code defined according to the ANSI and ODBC standards