Mysql数据库day05
行内视图子查询
l 子查询在from后面
l 从一个查询的查询结果，再查询
select … from (select …) t
l 行内视图后面必须起一个别名
Select字段列表中的子查询
l select a,b,c,(select …) from …
多表关联查询
l 多张表，按条件连接成一张表
l Select 表1.字段列表, 表2.字段列表
from 表1,表2
where 表1.字段1=表2.字段2
l 自连接：将一张表看作是两张表进行连接
l 标准表连接语法：
select …
from
表1
inner join 表2
on 连接条件
inner join 表3
on 连接条件
inner join 表4
on 连接条件
l 外连接
连接条件以外的数据，也查询出来
n 左外连接
a left outer join b on …
左侧a表，条件外的数据也查询出来
n 右外连接
a right outer join b on …
右侧b表，条件外的数据也查询出来
select查询结构
select
distinct
from
join on
left join on
right join on
where
group by
having
order by
limit
事务
l 事务是数据操作的最小单元
l 多个数据增删改操作，完成的一项业务处理
l 如果事务事务成功，其中每一项操作都生效
如果事务事务失败，其中每一项操作都失败
l 数据库数据操作，以事务为一个最小操作单元，
不应该以一个sql语句为一个操作单元；
要么整个事务成功，要么整个事务失败
l 在当前事务中对数据的修改，只对当前连接可见
l ACID
A - 原型性 Atomic
C - 一致性 Consistency
转账前 a+b = 100
转帐后 a+b = 100
I - 隔离性 Isolation
一个事物进行中时，
另一事物不能操作数据
D - 持久性 Durancy
提交事务之后，
数据持久生效
l 事务操作
n 开始事务
start transaction; / begin;
n 提交事务
commit;
n 回滚事务
rollback;
隔离级别
l set tx_isolation='read-uncommitted';
l set tx_isolation='read-committed';
l set tx_isolation='repeatable-read';
l set tx_isolation='serializable';
l 隔离级别越低，效率越高，数据访问冲突越多
l 隔离级别越高，效率越低，数据访问冲突越少
l 数据库默认隔离级别是repeatable-read
数据访问冲突问题
n 脏读
读取到其他事物未提交的数据
n 幻读
一个事务添加或删除数据并提交，
另一个事务查询不到新数据，或仍查询到已删除的数据
n 不可重复读
再次查询的数据，与第一次查询的数据不一致
隔离级别对应数据访问冲突现象
| read-uncommitted |
脏读，幻读，不可重复读 |
| read-committed |
幻读，不可重复读 |
| repeatable-read |
幻读 |
| serializable |
|
视图
l 将一个查询保存在数据库中
l 可以从这个查询的查询结果，再查询
l 作用：
n 简化查询
n 安全
可以让低权限用户，只能从视图查询，
而不能去碰真实数据表
创建视图
create (or replace) view v1
as
select …
查看视图
show tables;
desc v1;
show create table v1\G
删除视图
drop view v1;
索引
l 提高字段的过滤查询速度
l 常见索引数据结构：
n B-Tree
n 哈希表
创建索引
create index index_name on tb1(name);
n where name='abc'
n where name like 'abc%'
n order by name
n where name like '%abc%' 不使用索引
create index index_name on tb1(name, birthday);
n where name='abc' and birthday='xxxxxx'
n where name='abc'
n where birthday='xxxx' 第二个字段单独过滤不使用索引
查看索引
show create table tb1\G
删除索引
alter table tb1 drop index index_name;
练习
1. 只有一个下属的主管信息
主管id 手下
100 3
120 5
130 1
160 1
员工id first_name salary
130 xxx xxx
160 xxx xxx
select employee_id,first_name,salary
from employees where employee_id in
(select manager_id from employees
where manager_id is not null
group by manager_id having count(*)=1)
2. 平均工资最高的部门编号
过滤条件是用 9000 过滤部门
部门 平均工资
30 6000
50 7000
80 5000
90 9000
100 9000
select department_id,round(avg(salary),2) a
from employees
where department_id is not null
group by department_id
having a=
(select max(a) from
(select department_id, round(avg(salary),2) a
from employees
where department_id is not null
group by department_id) t) -- 必须起别名
3. 平均工资最低的工种，查询做这些工作的人
工种 平均工资
A 5000
B 9000
C 3000
D 12000
E 3000
Select employee_id,first_name,salary,job_id
From employees
Where job_id in
(select job_id from employees
group by job_id
having round(avg(salary),2)=
(select min(a) from
(select job_id,round(avg(salary),2) a
from employees
group by job_id) t))
4. 查询员工工资，同时列出最高工资
select
employee_id,
first_name,
salary,
(select max(salary) from employees) max
From
Employees;
5. 部门表
Select * from departments;
6. 查询员工信息，并显示部门名称
select e.employee_id,
e.first_name,
e.salary,
d.department_id,
d.department_name
from employees e,
departments d
where e.department_id=d.department_id;
7. 地区表
Select * from locations;
8. 查询部门，同时显示部门所在城市
select
d.department_id,
d.department_name,
L.location_id,
L.city
from
departments d,locations L
where
d.location_id=L.location_id;
9. 查询员工，显示部门名和城市
select
e.employee_id,e.first_name,e.salary,
d.department_name,
L.city
from
employees e,
departments d,
locations L
where
e.department_id=d.department_id
and
d.location_id=L.location_id;
10. 查询部门，显示部门经理名
select
d.department_id,
d.department_name,
d.manager_id,
e.first_name
from
departments d,
employees e
where
d.manager_id=e.employee_id;
11. 查询员工，显示主管名
select
e1.employee_id,e1.first_name,e1.salary,
e2.first_name manager
from
employees e1,employees e2
where
e1.manager_id=e2.employee_id;
12. 查询员工，显示主管名、部门、部门经理名、城市
employees e1
employees e2
departments d
employees e3
locations l
select e1.employee_id,e1.first_name,
e2.first_name mgr, d.department_name,
e3.first_name manager,l.city
from employees e1
join employees e2
on e1.manager_id=e2.employee_id
join departments d
on e1.department_id=d.department_id
join employees e3
on d.manager_id=e3.employee_id
join locations l
on d.location_id=l.location_id;
13. 查询所有员工，显示部门名，没有部门显示null
select e.employee_id,e.first_name,
d.department_name
from employees e
left join departments d
on e.department_id=d.department_id;
14. 按城市分组，计算每个城市的员工数量
select
l.city, count(*) c
from employees e
join departments d
on e.department_id=d.department_id
join locations l
on d.location_id=l.location_id
group by l.city
order by c desc;
15. Seattle 市所有的员工信息
select
e.employee_id,e.first_name,e.salary,
l.city
from employees e
join departments d
on e.department_id=d.department_id
join locations l
on d.location_id=l.location_id
where l.city='Seattle';
16. 事务测试
use test;
drop table if exists tb1;
create table tb1(
id int primary key auto_increment,
name varchar(20)| 回话1 |
回话2 |
| Use test |
User test |
| begin; |
begin; |
| Insert into tb1(name) values('aa'); |
|
| select * from tb1; |
select * from tb1; |
| commit; |
|
|
|
select * from tb1; |
|
|
commit; |
|
|
select * from tb1; |
);
| 回话1 |
回话2 |
| Use test |
Use test |
| begin; |
begin; |
| insert into tb1(name) values('bb'); |
|
| update tb1 set name='aaaa' where id=1; |
|
|
|
Select * from tb1; |
|
|
update tb1 set name='aaaaaaaa' where id=1; |
| rollback; |
|
| Select * from tb1; |
Select * from tb1; |
|
|
commit; |
17. 隔离级别测试
| 回话1 |
回话2 |
| set tx_isolation= 'read-uncommitted'; |
set tx_isolation= 'read-uncommitted'; |
| rollback; begin; |
rollback; begin; |
| Insert into tb1(name) values('bb'); |
|
|
|
Select * from tb1; |
| Update tb1 set name='a' where id=1; |
|
|
|
Select * from tb1; |
| rollback; |
|
|
|
Select * from tb1; |
| 回话1 |
回话2 |
| set tx_isolation= 'read-committed' |
set tx_isolation= 'read-committed' |
| rollback; begin; |
rollback; begin; |
| Insert into tb1(name) values('bb'); |
|
| Update tb1 set name='aaaaaaaaaaaa' where id=1; |
|
|
|
select * from tb1; |
| commit; |
|
|
|
select * from tb1; |
| 回话1 |
回话2 |
| set tx_isolation= 'repeatable-read' |
set tx_isolation= 'repeatable-read' |
| rollback; begin; |
rollback; begin; |
| insert into tb1(name) values('cc'); |
|
| update tb1 set name='bbbbbbbb' where id=4; |
|
|
|
select * from tb1; |
| commit; |
|
|
|
select * from tb1; |
|
|
update tb1 set name=concat('*',name); |
|
|
select * from tb1; |
| 回话1 |
回话2 |
| set tx_isolation= 'repeatable-read' |
set tx_isolation= 'repeatable-read' |
| rollback; begin; |
rollback; begin; |
|
|
select * from tb1; |
| delete from tb1 where id=4; |
|
|
|
select * from tb1; |
| commit; |
|
|
|
select * from tb1; |
|
|
update tb1 set name=concat('#',name); |
|
|
select * from tb1; |
18. 视图测试
use hr;
create or replace view v1 as
select
l.city, count(*) c
from employees e
join departments d
on e.department_id=d.department_id
join locations l
on d.location_id=l.location_id
group by l.city;
show tables;
desc v1;
show create table v1\G
select * from v1;
select c from v1;
select * from v1 where c=1;
练习
子查询:
1. 工资多于工种 IT_PROG 平均工资的员工
select employee_id,first_name,salary
from employees
where salary>(
select avg(salary) from employees
where job_id='IT_PROG'
);
2. 平均工资最高的工种, 显示工作全名
Jobs表保存工作全名
select job_id,job_title from jobs
where job_id in
(
select job_id from employees
group by job_id
having avg(salary)=(
select max(a) from
(select job_id,avg(salary) a from employees
group by job_id) t
)
);
3. 每个部门拿最高工资的人
select employee_id,first_name,salary,department_id
from employees
where (department_id,salary) in
(
select department_id,max(salary)
from employees
where department_id is not null
group by department_id
)
4. 每年第一个入职的人
select employee_id,first_name,salary,hire_date
from employees
where hire_date in
(
select min(hire_date)
from employees
group by extract(year from hire_date)
);
5. 平均工资最高的部门编号
select department_id,round(avg(salary),2) a
from employees
where department_id is not null
group by department_id
having a =
(
select max(a) from
(select department_id,round(avg(salary),2) a
from employees
where department_id is not null
group by department_id) t
);
6. 下属人数最多的人，查询其个人信息
select employee_id,first_name,salary
from employees
where employee_id in
(
Select manager_id
From employees
Where manager_id is not null
Group by manager_id
Having count(*)=
(
select max(c) from
(
Select manager_id, count(*) c
From employees
Where manager_id is not null
Group by manager_id
) t
)
)