Managing & Using MySQL, 2nd Edition

Metadata

The term metadata sounds officious, but it is really nothing more than extra data about some object that would otherwise waste resources if it were actually kept in the object. For example, simple applications do not need the name of the columns associated with a ResultSet —the programmer probably knew that when the code was written. Embedding this extra information in the ResultSet class is thus not considered by JDBC’s designers to be part of the core of ResultSet functionality. Data such as the column names, however, is very important to some database programmers—especially those writing dynamic database access. The JDBC designers provide access to this extra information—the metadata—via the ResultSetMetaData interface. This class specifically provides:

• The number of columns in a result set

• Whether NULL is a valid value for a column

• The label to use for a column header

• The name for a given column

• The source table for a given column

• The data type of a given column

Example 13-6 shows some of the source code from a command-line tool that accepts arbitrary user input and sends it to MySQL for execution. The rest of the code for this example can be found at the O’Reilly web site with the other examples from this book.

Example 13-6. An application for executing dynamic SQL

import java.sql.*; public class Exec { public static void main(String args[]) { Connection con = null; String sql = ""; for(int i=0; i<args.length; i++) { sql = sql + args[i]; if( i < args.length - 1 ) { sql ...