Avoiding Output Column Order Problems When Writing Programs

3.3.1 Problem

You're issuing a SELECT * query from within a program, and the columns don't come back in the order you expect.

3.3.2 Solution

When you use * to select columns, all bets are off; you can't assume anything about the order in which they'll be returned. Either name the columns explicitly in the order you want, or retrieve them into a data structure that makes their order irrelevant.

3.3.3 Discussion

The examples in the previous section illustrate the differences between using * versus a list of names to specify output columns when issuing SELECT statements from within the mysql program. The difference between approaches also may be significant when issuing queries through an API from within your own programs, depending on how you fetch result set rows. If you select output columns using *, the server returns them using the order in which they are listed in the table definitionan order that may change if the table structure is modified. If you fetch rows into an array, this non-determinacy of output column order makes it impossible to know which column each array element corresponds to. By naming output columns explicitly, you can fetch rows into an array with confidence that the columns will appear in the array in the same order that you named them in the query.

On the other hand, your API may allow you to fetch rows into a structure containing elements that are accessed by name. (For example, in Perl you can use a hash; in PHP you can use an associative array or an object.) If you do this, you can issue a SELECT * query and then access structure members by referring to the column names in any order you want. In this case, there is effectively no difference between selecting columns with * or by naming them explicitly: If you can access values by name within your program, their order within result set rows is irrelevant. This fact makes it tempting to take the easy way out by using SELECT * for all your queries, even if you're not actually going to use every column. Nevertheless, it's more efficient to name specifically only the columns you want so that the server doesn't send you information you're just going to ignore. (An example that explains in more detail why you may want to avoid retrieving certain columns is given in Recipe 9.9, in Recipe 9.9.10.")

Using the mysql Client Program

Writing MySQL-Based Programs

Record Selection Techniques

Working with Strings

Working with Dates and Times

Sorting Query Results

Generating Summaries

Modifying Tables with ALTER TABLE

Obtaining and Using Metadata

Importing and Exporting Data

Generating and Using Sequences

Using Multiple Tables

Statistical Techniques

Handling Duplicates

Performing Transactions

Introduction to MySQL on the Web

Incorporating Query Resultsinto Web Pages

Processing Web Input with MySQL

Using MySQL-Based Web Session Management

Appendix A. Obtaining MySQL Software

Appendix B. JSP and Tomcat Primer

Appendix C. References

MySQL Cookbook
MySQL Cookbook
ISBN: 059652708X
EAN: 2147483647
Year: 2005
Pages: 412
Authors: Paul DuBois

Flylib.com © 2008-2020.
If you may any questions please contact us: flylib@qtcs.net