Eliminating Duplicates by Adding an Index

8.9.1 Problem

A table has duplicates and you'd like to get rid of them.

8.9.2 Solution

One way to do this is to create a unique index on the column or columns containing duplicates.

8.9.3 Discussion

If MySQL discovers duplicate key values when you try to create a PRIMARY KEY or a UNIQUE index, it aborts the ALTER TABLE operation. To ignore the duplicates and proceed anyway, use ALTER IGNORE TABLE rather than ALTER TABLE. The IGNORE keyword tells MySQL to retain the first row containing a duplicated key value and discard the others. This is, in fact, a useful way to eliminate duplicates in a column or set of columns: just create a unique-valued index and let MySQL throw away the duplicates. (If you need to identify which key values are duplicated, though, that's not a suitable technique. See Recipe 14.4 for information on duplicate identification.)

To see how IGNORE works to eliminate duplicates, use mytbl, which now has no indexes if you've issued the index-modification statements shown earlier. First, insert some duplicate values into the table:

mysql> INSERT INTO mytbl (i,c) VALUES(1,'a'),(1,'a'),(1,NULL),(1,NULL),
 -> (2,'a'),(2,'a'),(2,'b'),(2,'b');
mysql> SELECT * FROM mytbl;
+---+------+
| i | c |
+---+------+
| 1 | a |
| 1 | a |
| 1 | NULL |
| 1 | NULL |
| 2 | a |
| 2 | a |
| 2 | b |
| 2 | b |
+---+------+

Now suppose you want to create a unique-valued index comprising the i and c columns. A PRIMARY KEY cannot be used here, because c contains NULL values. You can create a UNIQUE index, but if you try to do so without using IGNORE, you'll get an error:

mysql> ALTER TABLE mytbl ADD UNIQUE (i,c);
ERROR 1062 at line 1: Duplicate entry '1-a' for key 1

Add IGNORE to the statement, then use SELECT to have a look at the table contents to see how the duplicates have been removed:

mysql> ALTER IGNORE TABLE mytbl ADD UNIQUE (i,c);
mysql> SELECT * FROM mytbl;
+---+------+
| i | c |
+---+------+
| 1 | NULL |
| 1 | NULL |
| 1 | a |
| 2 | a |
| 2 | b |
+---+------+

The output shows that duplicate records have been eliminatedexcept those that contain NULL in the key values. That's because UNIQUE indexes allow multiple NULL values. For another technique that removes even duplicates that contain NULLs, see Recipe 14.7.

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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