count distinct 统计

以前是在Set中来做
现在可以在数据库中来做了
O(∩_∩)O哈哈~，是不是很爽啊？

表：

mysql> select userlevel,username from users;
+-----------+----------+
| userlevel | username |
+-----------+----------+
| admin     | cdliang  |
| service   | bobo     |
| user      | caocao   |
| user      | yongyong |
| service   | zhouzhou |
+-----------+----------+
5 rows in set (0.00 sec)

目的：查询不重复的数目

1.distinct

mysql> select count(distinct userlevel)权限数目 from users;
+----------+
| 权限数目 |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)


2.使用嵌套查询

mysql> select count(*)权限数目 from (select  distinct userlevel from users) t;
+----------+
| 权限数目 |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)