MySQL Cookbook

Chapter 9. Obtaining and Using Metadata

Introduction

Most of the queries used so far have been written to work with the data stored in the database. That is, after all, what the database is designed to hold. But sometimes you need more than just data values. You need information that characterizes or describes those values—that is, the query metadata. Metadata information is used most often in relation to processing result sets, but also is available for other aspects of your interaction with MySQL. This chapter describes how to obtain and use the following types of metadata:

• Information about the result of queries.

  When you delete or update a set of rows, you can determine the number of rows that were changed. For a SELECT query, you can find out the number of columns in the result set, as well as information about each column in the result set, such as the column name and its display width. Such information often is essential for processing the results. For example, if you’re formatting a tabular display, you can determine how wide to make each column and whether to justify values to the left or right.

• Information about tables and databases.

  Information pertaining to the structure of tables and databases is useful for applications that need to enumerate a list of tables in a database or databases hosted on a server (for example, to present a display allowing the user to select one of the available choices). You can also use this information to determine whether tables or databases ...