3.11 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.
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