MySQL Cookbook, 2nd Edition

Removing Duplicate Rows

Problem

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

Solution

Use DISTINCT.

Discussion

Some queries produce results containing duplicate rows. 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    |
+---------+

That result is heavily redundant. Adding DISTINCT to the query removes the duplicate rows, 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) |
+---------+----------+---------------+
|    2006 |        5 |            11 |
|    2006 |        5 |            12 |
|    2006 |        5 |            13 |
|    2006 |        5 |            14 |
|    2006 |        5 |            15 |
|    2006 |        5 |            16 |
|    2006 |        5 |            17 |
|    2006 |        5 |            19 |
+---------+----------+---------------+

To count the number of unique values in a column, use COUNT(DISTINCT):

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

See Also

Chapter 8 revisits DISTINCT and ...