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.