mysql> select * from Employee;
+------+-------------+
| id | description |
+------+-------------+
| 1 | Programmer |
| 2 | Tester |
| 3 | Tester |
| 4 | Manager |
| 5 | Tester |
| 6 | Tester |
| 7 | Manager |
| 8 | Tester |
+------+-------------+
1. 在count 不重复的记录的时候能用到,比如
SELECT COUNT( DISTINCT description) FROM tablename;
结果为:3
就是计算talbebname表中description不同的记录有多少条。