Removing Duplicate Rows

3.11.1 Problem

Output from a query contains duplicate records. You want to eliminate them.

3.11.2 Solution

Use DISTINCT.

3.11.3 Discussion

Some queries produce results containing duplicate records. For example, to see who sent mail, you could query the mail table like this:

mysql> SELECT srcuser FROM mail;
+---------+
| srcuser |
+---------+
| barb |
| tricia |
| phil |
| barb |
| gene |
| phil |
| barb |
| tricia |
| gene |
| phil |
| gene |
| gene |
| gene |
| phil |
| phil |
| gene |
+---------+

But that result is heavily redundant. Adding DISTINCT to the query removes the duplicate records, producing a set of unique values:

mysql> SELECT DISTINCT srcuser FROM mail;
+---------+
| srcuser |
+---------+
| barb |
| tricia |
| phil |
| gene |
+---------+

DISTINCT works with multiple-column output, too. The following query shows which dates are represented in the mail table:

mysql> SELECT DISTINCT YEAR(t), MONTH(t), DAYOFMONTH(t) FROM mail;
+---------+----------+---------------+
| YEAR(t) | MONTH(t) | DAYOFMONTH(t) |
+---------+----------+---------------+
| 2001 | 5 | 11 |
| 2001 | 5 | 12 |
| 2001 | 5 | 13 |
| 2001 | 5 | 14 |
| 2001 | 5 | 15 |
| 2001 | 5 | 16 |
| 2001 | 5 | 17 |
| 2001 | 5 | 19 |
+---------+----------+---------------+

To count the number of unique values, do this:

mysql> SELECT COUNT(DISTINCT srcuser) FROM mail;
+-------------------------+
| COUNT(DISTINCT srcuser) |
+-------------------------+
| 4 |
+-------------------------+

COUNT(DISTINCT) requires MySQL 3.23.2 or higher.

3.11.4 See Also

DISTINCT is revisited in Chapter 7. Duplicate removal is discussed in more detail in Chapter 14.

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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