数据库查询一些小练习和注意事项

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表一 student

表结构

mariadb root@localhost:bit_student> desc student
                                 -> ;
+---------+-------------+------+-----+---------+----------------+
| Field   | Type        | Null | Key | Default | Extra          |
+---------+-------------+------+-----+---------+----------------+
| id      | int(11)     | NO   | PRI | <null>  | auto_increment |
| name    | varchar(20) | NO   |     |         |                |
| chinese | float(3,1)  | NO   |     | 0.0     |                |
| english | float(3,1)  | NO   |     | 0.0     |                |
| math    | float(3,1)  | NO   |     | 0.0     |                |
+---------+-------------+------+-----+---------+----------------+
                                 -> ;
+----+----------+---------+---------+------+
| id | name     | chinese | english | math |
+----+----------+---------+---------+------+
| 1  | 李涛     | 89.0    | 78.0    | 90.0 |
| 2  | 唐僧     | 67.0    | 98.0    | 56.0 |
| 3  | 孙悟空   | 87.0    | 78.0    | 77.0 |
| 4  | 老妖婆   | 88.0    | 98.0    | 90.0 |
| 5  | 红孩儿   | 82.0    | 84.0    | 67.0 |
| 6  | 如来佛祖 | 55.0    | 85.0    | 45.0 |
| 7  | 菩萨     | 75.0    | 65.0    | 30.0 |
+----+----------+---------+---------+------+

表查询

> 1.查询 总分大于200分 并且 数学成绩小于语文成绩 的 姓唐 的学生

mariadb root@localhost:bit_student> select id,name,(chinese+english+math) as total
                                 -> from student
                                 -> where (chinese+english+math) >200 and math < chinese
                                 -> and name like '唐%';
+----+------+-------+
| id | name | total |
+----+------+-------+
| 2  | 唐僧 | 221.0 |
+----+------+-------+
1 row in set
Time: 0.013s

注意 as别名只对查询结果有效,所以where中还得写(chinese+english+math)而不是total,
如果需要使用通配符,使用like而不是=,而且如果不使用通配符时,虽然like和=等效,但是=更高效(可以忽略不计)

---

>2.查询英语分数在80 - 90 之间的同学

mariadb root@localhost:bit_student> select id,name,english
                                 -> from student 
                                 -> where english between 80 and 90; 
+----+----------+---------+
| id | name     | english |
+----+----------+---------+
| 5  | 红孩儿   | 84.0    |
| 6  | 如来佛祖 | 85.0    |
+----+----------+---------+

注意事项 在mysql 中between 是闭区间,not between是开区间,还有在日期中
想查询某一天的日期时

between '2017-07-25 00:00:00' and  '2017-07-25 24:00:00';

后面那个日期实际相当于 ‘2017-07-25 00:00:00’ and前后是一个时间,所以应该改为

between '2017-07-25 00:00:00' and  '2017-07-26 00:00:00';

> 3.查询数学成绩为89,90,91的同学

mariadb root@localhost:bit_student> select id,name,math
                                 -> from student
                                 -> where math in (89,90,91);
+----+--------+------+
| id | name   | math |
+----+--------+------+
| 1  | 李涛   | 90.0 |
| 4  | 老妖婆 | 90.0 |
+----+--------+------+
2 rows in set
Time: 0.009s

in 多用于多表查询的内连接

>4.要求显示student表中的信息，显示格式：“XXX的语文是XXX分，数学XXX分，英语XXX分

mariadb root@localhost:bit_student> select concat(name,'的语文',chinese,'分,数学',math,'分
                                 -> ,英语',english,'分') as '通知单'
                                 -> from student;
+--------------------------------------------+
| 通知单                                     |
+--------------------------------------------+
| 李涛的语文89.0分,数学90.0分,英语78.0分     |
| 唐僧的语文67.0分,数学56.0分,英语98.0分     |
| 孙悟空的语文87.0分,数学77.0分,英语78.0分   |
| 老妖婆的语文88.0分,数学90.0分,英语98.0分   |
| 红孩儿的语文82.0分,数学67.0分,英语84.0分   |
| 如来佛祖的语文55.0分,数学45.0分,英语85.0分 |
| 菩萨的语文75.0分,数学30.0分,英语65.0分     |
+--------------------------------------------+
7 rows in set
Time: 0.009s

这个不太常用 知道就好

5>.对数学成绩进行排序

mariadb root@localhost:bit_student> select * from student order by math desc;
+----+----------+---------+---------+------+
| id | name     | chinese | english | math |
+----+----------+---------+---------+------+
| 1  | 李涛     | 89.0    | 78.0    | 90.0 |
| 4  | 老妖婆   | 88.0    | 98.0    | 90.0 |
| 3  | 孙悟空   | 87.0    | 78.0    | 77.0 |
| 5  | 红孩儿   | 82.0    | 84.0    | 67.0 |
| 2  | 唐僧     | 67.0    | 98.0    | 56.0 |
| 6  | 如来佛祖 | 55.0    | 85.0    | 45.0 |
| 7  | 菩萨     | 75.0    | 65.0    | 30.0 |
+----+----------+---------+---------+------+

降序 desc

6> 统计一个班有多少学生

mariadb root@localhost:bit_student> select count(*)as '人数'  from student;
+------+
| 人数 |
+------+
| 7    |
+------+
1 row in set
Time: 0.006s

count(*) 是统计这张表的列数

也可以使用coutn(列名)
但注意会排除此列为NULL的情况

>7.计一个班各科加起来的成绩

mariadb root@localhost:bit_student> select sum(chinese) as '语文',sum(math) as '数学',sum(
                                 -> english) as '英语'  from student;
+-------+-------+-------+
| 语文  | 数学  | 英语  |
+-------+-------+-------+
| 543.0 | 455.0 | 586.0 |
+-------+-------+-------+
1 row in set
Time: 0.005s

>7.计一个班各科平均成绩

mariadb root@localhost:bit_student> select avg(chinese) as '语文',avg(math) as '数学',avg
                                 -> (english) as '英语'  from student;
+----------+------+----------+
| 语文     | 数学 | 英语     |
+----------+------+----------+
| 77.57143 | 65.0 | 83.71429 |
+----------+------+----------+
1 row in set
Time: 0.008s

>8 统计数学成绩高于平均成绩的同学的姓名和数学成绩

mariadb root@localhost:bit_student> select name,math from student
                                 -> having math > avg(math);
+------+------+
| name | math |
+------+------+
| 李涛 | 90.0 |
+------+------+
1 row in set
Time: 0.007s

having是在结果集里进行筛选,而
where子句是筛选出结果集,
在sql中语句的执行顺讯

from-->where-->group by -->having --- >order by --> select;
from :确定表
where:在表中筛选出特定的结果集
group by 对结果集进行按列排序
having 在结果集中再次筛选
select 在结果集返回需求数据

聚集函数 avg sum 等只能在结果集中进行,个人猜想是如果对全表进行这些函数会严重影响性能.(找个理由就好记忆了..)
统计数学成绩高于平均成绩的同学的姓名和数学成绩 还有一种写法 就是in的子查询

sql
mariadb root@localhost:bit_student> select name from student where
-> student.math >
-> (select avg(math) from student);
+--------+
| name |
+--------+
| 李涛 |
| 孙悟空 |
| 老妖婆 |
| 红孩儿 |
+--------+


表2 Oracle经典的SOCCT数据库

表结构

表查询