第7节-MySQL数据查询

1、指定列查询

1.1、查询所有记录

select * from student;

1.2、查询学号、学生名字

select sno,sname from student;

1.3、定义列的别名

select sno as 学号,sname as 学生名字 from student;
+--------+--------------+
| 学号   | 学生名字     |
+--------+--------------+
| 95001  | 洛燕妮       |
| 95002  | 欧阳炎       |

1.4、查询并去重处理

select distinct sno from student;

1.5、查询第5,6条记录

select sno from student limit 4,2;

1.6、查询前10条记录

select sno from student limit 10;

1.7、排序查询

--  asc 升序
--  desc 降序
select * from student order by sno desc; 

2、条件查询

2.1、查询操作符

2.2、常见查询方法

2.2.1、=

-- =
select * from student where sno='95001';

2.2.2、>= <=

-- >= <=
select * from student where age>=20 and age<=25;

2.2.3、is not null

-- is not null
select * from student where age is not null;

2.2.4、between and

-- between and
select * from student where age between 18 and 21;

2.2.5、in

-- in
select * from student where sno in('95002','95003');

2.2.6、like

-- like
select * from student where sname like '欧%';

3、regexp运算符

3.1、 regepx常用的查询

-- regexp 以什么结尾
select * from student where sname regexp '升$';

-- regepx 以95、94、93开头
select * from student where sno regexp '^9[543]';

4、统计查询

4.1、时间的运算

-- 增加生日的字段
alter table student add birthday datetime;

-- 插入一条生日的数据
insert into student values('95007','test','','20','计算机系','2002-01-02 12:30:12');

-- now函数
select now();
+---------------------+
| now()               |
+---------------------+
| 2022-11-10 22:54:16 |
+---------------------+

-- year、month、day、hour、minute、second函数
select year(birthday) from student;

-- 计算年龄
select year(now())-year(birthday) from student;

5、聚合函数

MySQL中提供5个聚合函数
avg -- 平均值
maxmin -- 最大值、最小值
sum -- 求和
count -- 计数

5.1、avg

-- 平均值
select avg(age) from student;

5.2、max、min

-- 最大值、最小值
select max(age),min(age) from student;

5.3、sum

-- 求和
select sum(age) from student;

5.4、count

-- 计数
select count(*) from student;

6、分组统计

6.1、group by

-- group by 按性别分组
select ssex,count(*) from student group by ssex;

6.2、having 

-- having 按部分分组并且把结果大于4取出
select sdept,count(*) from student group by sdept having count(*)>4;

6.3、with rollup

-- with rollup 合计
select sdept,count(*) from student group by sdept with rollup  having count(*)>0;

6.4、group_concat

6.4.1、去重,分组统计,列出具体的数量

-- group_concat,去重,分组统计,列出具体的数量
select sdept,count(distinct age) as 去重后部门年龄数,group_concat(distinct age) as 列出去重年龄具体数据 from student group by sdept;
+----------------------------------+--------------------------+--------------------------------+
| sdept                            | 去重后部门年龄数         | 列出去重年龄具体数据           |
+----------------------------------+--------------------------+--------------------------------+
| 020-电子信息工程系               |                        3 | 18,19,21                       |
| 学院-020-电子信息工程系          |                        1 | 25                             |
| 计算机系                         |                        1 | 20                             |
| 软件技术                         |                        1 | 29                             |
+----------------------------------+--------------------------+--------------------------------+

6.4.2、分组统计,列出具体的数量

-- group_concat,分组统计,列出具体的数量
select sdept,count(age) as 去重后部门年龄数,group_concat(age) as 列出去重年龄具体数据 from student group by sdept;
+----------------------------------+--------------------------+--------------------------------+
| sdept                            | 部门年龄数         | 列出年龄具体数据           |
+----------------------------------+--------------------------+--------------------------------+
| 020-电子信息工程系               |                        4 | 19,21,18,21                    |
| 学院-020-电子信息工程系          |                        1 | 25                             |
| 计算机系                         |                        1 | 20                             |
| 软件技术                         |                        1 | 29                             |
+----------------------------------+--------------------------+--------------------------------+

7、多表查询

连接查询分类:inner join[内连接]outer joinright joinleft joinfull join[联合查询]cross join[外连接]

7.1、测试表

-- 班级表
create table student(
    sno varchar(10),
    name varchar(20),
    departid int
);

insert into student values('1000','张三',1),('1001','李四',2),('1002','王五',3),('1003','老王',4);

-- 部门表
create table department(
    departid int primary key,
    departedname varchar(20)
);
insert into department values(1,'学生会'),(2,'计算机协会');

7.2、内连接【inner join】默认join

两张表在进行连接时,连接列字段的名称可以不同,但要求必须具有相同数据类型,长度和精度,且表达同一范畴的意义,通常连接列字段一般是数据表的主键和外键。

7.2.1、inner join 内连接

select s.sno,s.name,d.departedname from student as s inner join department as d on s.departid=d.departid;

7.2.2、inner join 内连接 + where

select s.sno,s.name,d.departedname from student as s inner join department as d on s.departid=d.departid where s.name='张三';

7.2.3、inner join + using

-- 当连接条件是由两张表相同名称且类型也相同的字段相连时,可以使用USIN
select s.sno,s.name,d.departedname from student as s inner join department as d using(departid);

7.3、外连接【outer join】

7.3.1、左外连接[left join]

-- left join 右边不匹配,则为null
select * from student as s left join department as d on s.departid=d.departid;
+------+--------+----------+----------+-----------------+
| sno  | name   | departid | departid | departedname    |
+------+--------+----------+----------+-----------------+
| 1000 | 张三   |        1 |        1 | 学生会          |
| 1001 | 李四   |        2 |        2 | 计算机协会      |
| 1002 | 王五   |        3 |     NULL | NULL            |
| 1003 | 老王   |        4 |     NULL | NULL            |
+------+--------+----------+----------+-----------------+

7.3.2、右外连接[right join]

-- right join ,左边不匹配,则null
select * from department as d right join student as s on s.departid=d.departid;
+----------+-----------------+------+--------+----------+
| departid | departedname    | sno  | name   | departid |
+----------+-----------------+------+--------+----------+
|        1 | 学生会          | 1000 | 张三   |        1 |
|        2 | 计算机协会      | 1001 | 李四   |        2 |
|     NULL | NULL            | 1002 | 王五   |        3 |
|     NULL | NULL            | 1003 | 老王   |        4 |
+----------+-----------------+------+--------+----------+

7.4、完连接[full join]

-- full join ,交叉链接,两边数据相乘
select * from student full join department;
+------+--------+----------+----------+-----------------+
| sno  | name   | departid | departid | departedname    |
+------+--------+----------+----------+-----------------+
| 1000 | 张三   |        1 |        1 | 学生会          |
| 1000 | 张三   |        1 |        2 | 计算机协会      |
| 1001 | 李四   |        2 |        1 | 学生会          |
| 1001 | 李四   |        2 |        2 | 计算机协会      |
| 1002 | 王五   |        3 |        1 | 学生会          |
| 1002 | 王五   |        3 |        2 | 计算机协会      |
| 1003 | 老王   |        4 |        1 | 学生会          |
| 1003 | 老王   |        4 |        2 | 计算机协会      |
+------+--------+----------+----------+-----------------+

 

7.5、联合查询 union 或 union all

7.5.1、准备测试数据

-- 联合查询的注意事项:
-- 1﹑两个查询的列数目必须相同
-- 2﹑并且对应列的数据类型相互兼容

-- 增加老师表,用于union,演示
create table teacher(
    sno varchar(10),
    name varchar(20),
    departid int
);

insert into student values('8000','张老师',1),('8001','李老师',2);

7.5.2、union 

-- 按字段联合查询 union distinct 会去重显示
select sno,name,departid from student 
union 
select sno,name,departid from teacher;

7.5.3、union all 

-- 按所有字段联合查询,不用去重
select * from student 
union all
select * from teacher;

8、子查询

8.1、集合查询

8.1.1、in

-- in 包含 
select * from student where age in (21,19);

8.1.2、not in

-- not in 不包含 
select * from student where age not in (21,19);

8.1.3、any|some

-- any或some 年龄值大于21或19,则查询出来
select * from student where age>any(select distinct age from student where age in (21,19));

8.1.4、all

-- all 年龄的值大于21且19,则查询出来
elect * from student where age>all(select distinct age from student where age in (21,19));

8.1.5、exists

-- exists 查询学生表在课程表有记录的学生
select * from student where exists(select * from score where student.sno=score.sno);

8.2、子查询-增

-- 复制表结构
create table stu_exists like student;

-- 查询结果,并且写入表格中
insert into stu_exists select * from student where sno in(select distinct sno from score);

8.3、子查询-删

-- 删除学生表信息
delete from student where sno in (select distinct sno from score);

8.4、子查询-改

-- 修改成绩表
update score set grade=85 where sno=(select sno from student where sno='95001') and courseid=1;

8.5、子查询-查

-- 查询有成绩的学生信息
select * from student where sno in(select distinct sno from score);
posted @ 2022-12-04 15:41  小粉优化大师  阅读(32)  评论(0编辑  收藏  举报