WHERE Clauses and Column Aliases

3.8.1 Problem

You want to refer to a column alias in a WHERE clause.

3.8.2 Solution

Sorry, you cannot.

3.8.3 Discussion

You cannot refer to column aliases in a WHERE clause. Thus, the following query is illegal:

mysql> SELECT t, srcuser, dstuser, size/1024 AS kilobytes
 -> FROM mail WHERE kilobytes > 500;
ERROR 1054 at line 1: Unknown column 'kilobytes' in 'where clause'

The error occurs because aliases name output columns, whereas a WHERE clause operates on input columns to determine which rows to select for output. To make the query legal, replace the alias in the WHERE clause with the column or expression that the alias represents:

mysql> SELECT t, srcuser, dstuser, size/1024 AS kilobytes
 -> FROM mail WHERE size/1024 > 500;
+---------------------+---------+---------+-----------+
| t | srcuser | dstuser | kilobytes |
+---------------------+---------+---------+-----------+
| 2001-05-14 17:03:01 | tricia | phil | 2338.36 |
| 2001-05-15 10:25:52 | gene | tricia | 975.13 |
+---------------------+---------+---------+-----------+

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

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