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mysql> use test_1
Database changed
mysql> select * from student;
ERROR 1146 (42S02): Table 'test_1.student' doesn't exist
mysql> select * from students;
+--------+----------+---------+
| stu_id | stu_name | stu_sex |
+--------+----------+---------+
| 1 | xiaobai | F |
+--------+----------+---------+
1 row in set (0.01 sec)

mysql> select * from students;
+--------+----------+---------+
| stu_id | stu_name | stu_sex |
+--------+----------+---------+
| 1 | xiaobai | F |
| 2 | xiaobai | F |
| 3 | xiaobai | F |
| 4 | xiao | F |
| 5 | xiaoming | F |
+--------+----------+---------+
5 rows in set (0.00 sec)

mysql> select stu_id, stu_name, stu_sex from students group by stu_name;
+--------+----------+---------+
| stu_id | stu_name | stu_sex |
+--------+----------+---------+
| 4 | xiao | F |
| 1 | xiaobai | F |
| 5 | xiaoming | F |
+--------+----------+---------+
3 rows in set (0.01 sec)

mysql> select distinct stu_name from students;
+----------+
| stu_name |
+----------+
| xiaobai |
| xiao |
| xiaoming |
+----------+
3 rows in set (0.00 sec)

mysql> select distinct stu_name, stu_id, stu_sex from students;
+----------+--------+---------+
| stu_name | stu_id | stu_sex |
+----------+--------+---------+
| xiaobai | 1 | F |
| xiaobai | 2 | F |
| xiaobai | 3 | F |
| xiao | 4 | F |
| xiaoming | 5 | F |
+----------+--------+---------+
5 rows in set (0.00 sec)

mysql> select *, count(distinct stu_name) from students group by stu_name;
+--------+----------+---------+--------------------------+
| stu_id | stu_name | stu_sex | count(distinct stu_name) |
+--------+----------+---------+--------------------------+
| 4 | xiao | F | 1 |
| 1 | xiaobai | F | 1 |
| 5 | xiaoming | F | 1 |
+--------+----------+---------+--------------------------+
3 rows in set (0.01 sec)

mysql>

目的：从表中筛选出不重复的

1.从表中仅仅将某一列的不重复项给筛选出来
select distinct stu_name from students;
仅仅将students表中stu_name列里不重复的项给筛选出来

2.从表中将某一列的不重复项给筛选出来，同时其关联的其它列也一起显示出来
select stu_id, stu_name, stu_sex from students group by stu_name;
使用的是group by
note：重复项保留的是最先出现的那一行

3.select *, count(distinct stu_name) from students group by stu_name;
会在输出的地方添加count（distinct stu_name)列

4.select distinct stu_name, stu_id, stu_sex from students;
会改变列的列号

5.select *, count(distinct stu_name) from students;

mysql> select *, count(distinct stu_name) from students;
+--------+----------+---------+--------------------------+
| stu_id | stu_name | stu_sex | count(distinct stu_name) |
+--------+----------+---------+--------------------------+
| 1 | xiaobai | F | 3 |
+--------+----------+---------+--------------------------+
1 row in set (0.00 sec)