【5】查询练习：DISTINCT、Between...and...、in、order by、count

1.查询student表中所有字段：

mysql> select * from student;

2.查询student表中指定某些字段：

mysql> select stu_name,stu_sex,class from student;

3.去重：DISTINCT

mysql> select cour_num from score;
+----------+
| cour_num |
+----------+
| 1-245    |
| 1-245    |
| 1-245    |
| 2-271    |
| 2-271    |
| 2-271    |
| 3-105    |
| 4-321    |
+----------+
8 rows in set (0.00 sec)

mysql> select distinct cour_num from score;
+----------+
| cour_num |
+----------+
| 1-245    |
| 2-271    |
| 3-105    |
| 4-321    |
+----------+
4 rows in set (0.00 sec)

4.查询区间：BETWEEN ... AND...

mysql> select * from score where degree between 60 and 80;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11426   | 1-245    |     61 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

或运算符比较：

mysql> select * from score where degree > 60 and degree < 80;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11426   | 1-245    |     61 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

5.表示<或者关系>的查询：in

查询score表中成绩为75，82或84的记录：

mysql> select * from score where degree in(75,82,84);
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11423   | 1-245    |     84 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11426   | 2-271    |     82 |
+---------+----------+--------+

6.查询student表中<班级为'113'>或<性别为'M'>的同学记录：

表示或者关系：or

mysql> select * from student where class='113' or stu_sex='M';
+---------+----------+---------+---------------------+-------+
| stu_num | stu_name | stu_sex | stu_birth           | class |
+---------+----------+---------+---------------------+-------+
| 11328   | DingQi   | F       | 1994-08-15 00:00:00 | 113   |
| 11423   | Bob      | M       | 1998-04-25 00:00:00 | 114   |
| 11424   | LinJie   | M       | 1994-06-12 00:00:00 | 114   |
| 11425   | XieZhou  | M       | 1995-03-11 00:00:00 | 114   |
+---------+----------+---------+---------------------+-------+

7.以class降序查询student表中的所有记录（默认就是升序了）：

mysql> select * from student order by class desc;
+---------+----------+---------+---------------------+-------+
| stu_num | stu_name | stu_sex | stu_birth           | class |
+---------+----------+---------+---------------------+-------+
| 11422   | Baker    | F       | 1999-09-22 00:00:00 | 114   |
| 11423   | Bob      | M       | 1998-04-25 00:00:00 | 114   |
| 11424   | LinJie   | M       | 1994-06-12 00:00:00 | 114   |
| 11425   | XieZhou  | M       | 1995-03-11 00:00:00 | 114   |
| 11426   | MingHui  | F       | 1998-08-09 00:00:00 | 114   |
| 11427   | NanNan   | F       | 1995-10-20 00:00:00 | 114   |
| 11328   | DingQi   | F       | 1994-08-15 00:00:00 | 113   |
| 11215   | JiaWei   | F       | 1993-07-28 00:00:00 | 112   |
+---------+----------+---------+---------------------+-------+

以（order by）成绩降序（desc）查询：

mysql> select * from score order by degree desc;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
| 11425   | 2-271    |     89 |
| 11423   | 1-245    |     84 |
| 11426   | 2-271    |     82 |
| 11427   | 1-245    |     78 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11426   | 1-245    |     61 |
+---------+----------+--------+

以（order by）成绩升序（asc）查询：

mysql> select * from score order by degree asc;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11426   | 1-245    |     61 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11427   | 1-245    |     78 |
| 11426   | 2-271    |     82 |
| 11423   | 1-245    |     84 |
| 11425   | 2-271    |     89 |
| 11422   | 3-105    |     92 |
+---------+----------+--------+

8.以cour_num升序，degree降序查询score表中的所有记录：

mysql> select * from score;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
| 11423   | 1-245    |     84 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11425   | 2-271    |     89 |
| 11426   | 1-245    |     61 |
| 11426   | 2-271    |     82 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

mysql> select * from score cour_num asc,degree desc;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'asc,degree desc' at line 1
mysql> select * from score order by cour_num asc,degree desc;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11423   | 1-245    |     84 |
| 11427   | 1-245    |     78 |
| 11426   | 1-245    |     61 |
| 11425   | 2-271    |     89 |
| 11426   | 2-271    |     82 |
| 11423   | 2-271    |     75 |
| 11422   | 3-105    |     92 |
| 11424   | 4-321    |     75 |
+---------+----------+--------+

9.查询'114'班的学生人数：

统计：count

mysql> select count(*) from student where class='114';
+----------+
| count(*) |
+----------+
|        6 |
+----------+

count(*):对表中所有的数目进行计数，不管表中包含的是空值还是非空值。

count(column):指定列时，对有值的行计数，忽略NULL值。

mysql> select * from pet;
+----------+-------------+---------+------+------------+------------+
| name     | owner       | species | sex  | birth      | death      |
+----------+-------------+---------+------+------------+------------+
| Puffball | Diane       | hamster | f    | 2000-09-22 | NULL       |
| Cuihua   | ZhouXingChi | DOG     | M    | 1999-09-21 | NULL       |
| DNN      | Huimin      | Cat     | f    | 2018-07-07 | NULL       |
| Claws    | Gwen        | cat     | m    | 1994-03-17 | NULL       |
| Buffy    | Harold      | dog     | f    | 1989-05-13 | NULL       |
| Fang     | Benny       | dog     | m    | 1909-08-27 | NULL       |
| Bowser   | Diane       | dog     | m    | 1979-08-31 | 1995-07-29 |
| Chirpy   | Gwen        | bird    | f    | 1989-09-01 | NULL       |
| Slim     | Benny       | snake   | m    | 2006-04-21 | NULL       |
| Puffball | Diane       | hamster | f    | 2009-10-12 | NULL       |
+----------+-------------+---------+------+------------+------------+

mysql> select count(name) from pet;
+-------------+
| count(name) |
+-------------+
|          10 |
+-------------+

mysql> select count(death) from pet;
+--------------+
| count(death) |
+--------------+
|            1 |
+--------------+

10.查询score表中最高分的学生学号和课程号：

mysql> select stu_num,cour_num from score where degree=(select max(degree) from score);
+---------+----------+
| stu_num | cour_num |
+---------+----------+
| 11422   | 3-105    |
+---------+----------+

其中：

mysql> select * from score;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
| 11423   | 1-245    |     84 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11425   | 2-271    |     89 |
| 11426   | 1-245    |     61 |
| 11426   | 2-271    |     82 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

 首先先找到最高分：

mysql> select max(degree) from score;
+-------------+
| max(degree) |
+-------------+
|          92 |
+-------------+

然后找最高分的学号和课程号：

mysql> select stu_num,cour_num from score where degree=(select max(degree) from score);
+---------+----------+
| stu_num | cour_num |
+---------+----------+
| 11422   | 3-105    |
+---------+----------+

排序的做法：

按degree排序，默认为升序：

mysql> select stu_num,cour_num,degree from score order by degree;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11426   | 1-245    |     61 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11427   | 1-245    |     78 |
| 11426   | 2-271    |     82 |
| 11423   | 1-245    |     84 |
| 11425   | 2-271    |     89 |
| 11422   | 3-105    |     92 |
+---------+----------+--------+

mysql> select stu_num,cour_num,degree from score order by degree limit 0,1;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11426   | 1-245    |     61 |
+---------+----------+--------+

可见，上述步骤是取出了第一行。（从0开始查入一条）

mysql> select stu_num,cour_num,degree from score order by degree desc limit 0,1;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
+---------+----------+--------+

降序排列，再取出第一行。就是所求的最高分所对应的序号和课程号了。

但是用这种排序的方法，可能取得最高分的同时有多个人那，所以也是有缺陷的。

mysql> select stu_num,cour_num,degree from score order by degree desc limit 0,2;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
| 11425   | 2-271    |     89 |
+---------+----------+--------+

mysql> select stu_num,cour_num,degree from score order by degree desc limit 1,3;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11425   | 2-271    |     89 |
| 11423   | 1-245    |     84 |
| 11426   | 2-271    |     82 |
+---------+----------+--------+

limit中第一个数是从哪个位置开始查，第二个数字是查多少条。