MySQL连接查询,子查询语句
语法:
select 查询列表
from 表1 别名 [连接类型]
jion 表2 别名
on 连接条件
[where 筛选条件]
[group by 分组]
[having 筛选条件]
[order by 排序列表]
分类:
内连接:inner
外连接:
左外:left [outer]
右外:right[outer]
全外:full[outer]
交叉连接:cross
#############(一)、内连接########################################
语法:
select 查询列表
from 表1 别名
ineer join 表2 别名
on 连接条件;(后可外加其他如where、group by等子句)
特点:
① 可添加排序、分组、筛选
② inner可以省略
③ 筛选条件放在where 后面,连接条件放在on后面,提高分离性,便于阅读(对比与sql92,它的筛选条件和连接条件都是放在where后面)
分类:
###等值连接
#案例1:查询员工名、部门名
select
last_name,
department_name
from
employees e
inner join departments d on e.department_id = d.department_id;
#案例2:查询名字中包含e的员工名和工种名【筛选】
select
last_name,
job_title
from
employees e
inner join jobs j on e.job_id = j.job_id
where
e.last_name like '%e%';
#案例3:查询那个部门个数大于3的部门名和员工个数,并按个数降序【排序】
select
department_name,
count(*)
from
employees e
inner join departments d on e.department_id = d.department_id
group by
department_name
having
count(*) > 3
order by
count(*) desc;
#案例4:查询部门个数大于3的城市名和部门个数【分组+筛选】
select
city,
count(*)
from
locations l
inner join departments d on l.location_id = d.location_id
group by
city
having
count(*) > 3;
#案例5:查询员工名、部门名、工种名、并按部门名降序
select
last_name,
department_name,
job_title
from
employees e
inner join departments d on e.department_id = d.department_id
inner join jobs j on j.job_id = e.job_id
order by
department_name desc;
###非等值连接
#查询员工的工资级别
select
*
from
job_grades;
select
grade_level,
last_name,
salary
from
employees e
join job_grades j on e.salary between lowest_sal
and highest_sal;
#查询工资级别的个数大于20,并且按工资级别降序【排序+分组+筛选】
select
count(*) 个数,
grade_level
from
employees e
join job_grades j on e.salary between lowest_sal
and highest_sal
group by
grade_level
having
count(*) > 20
order by
grade_level desc;
###自连接
#案例:查询员工的名字中包含'k'的员工名、上级的名字
select
e.last_name 员工名,
m.last_name 上级领导名
from
employees e
join employees m on e.manager_id = m.employee_id
where
e.last_name like '%k%';
###(二)、外连接#########################################
特点:
- 外连接的查询结果为主表中的所有记录
a) 如果从表中有和它匹配的,则显示为匹配的值
b) 如果从表中没有和它匹配的,则线束null
c) 外连接查询结果=内连接查询结果+主表中有而从表中没有的记录
- 左外连接,left join左边的是主表(右外连接,right join右边的为主表)
- 左外和右外交换两个表的顺序,可以实现同样的效果
- 全外连接=内连接查询结果+1表中有而表2中没有的记录+表2有而表1没有的记录
引入:
#查询没有男朋友的女神名
use girls;
select
*
from
beauty;
select
name,
bo.id
from
beauty bea
left join boys bo on bea.boyfriend_id = bo.id
where
bo.id is null;
###左(右)外连接
#查询哪个部门没有员工
#左外
select
department_name,
employee_id
from
departments d
left join employees e on d.department_id = e.department_id
where
employee_id is null;
-----------------------------------------------------------
#右外
select
department_name,
employee_id,
from
employees e
right join departments d on d.department_id = e.department_id
where
employee_id is null;
###全外连接
#查询女神表和男朋友表的全外连接
select b.*,bo.*
from beauty b
full outer join boys bo
on b.boyfriend_id = bo.id;
###交叉连接(两个表进行笛卡尔乘积)
select b.*,bo.*
from beauty b
cross join boys bo;
总结(sql92 pk sql99)
功能:sql99支持的较多
可读性:sql99实现连接查询的条件和筛选选条件的分离,可读性较高
十八、子查询
含义:出现在其他语句中的select语句,称为子查询或内查询
外部的查询语句 ,称为主查询或外查询
分类:
按子查询出现的位置:
select 后面(仅仅支持标量子查询)
from 后面(支持表子查询)
where 或having后面(标量子查询、列子查询、行子查询)-》重要
exists后面(相关子查询)(表子查询)
按结果集的行列数不同:
标量子查询(结果集只有一行一列)
列子查询(结果集只有一列多行)
行子查询(结果集有一行多列)
表子查询(结果一般为多行多列)
###where或having 后面
1. 标量子查询(单行子查询)
2. 列子查询(多行子查询)
3. 行子查询(多行多列)
特点:
① 子查询放在小括号内
② 子查询一般放在条件的右侧
③ 标量子查询,一般搭配着单行操作符使用(> < <> <= >= = )
④ 列子查询,一般搭配着多行操作符使用(in 、 any|some 、 all)
⑤ 子查询的执行优先于主查询的执行,主查询的结果用到了子查询的结果
###标量子查询
#案例1:谁的工资比abel高?
select *
from employees
where salary > (
select salary from employees where last_name = 'abel' #先查出abel的工资
);
#案例2:返回job_id与141号相同,salary比143号员工多的员工、姓名、job_id和工资
select last_name,job_id,salary
from employees
where job_id = (
select job_id from employees where employee_id = 141
) and salary>(
select salary from employees where employee_id = 143
);
#案例3:返回工资最少的员工的last_name job_id salary
select last_name,job_id,salary
from employees
where salary =(
select min(salary) from employees
);
#案例4:查询最低工资大于50号部门最低工资的部门id和其最低工资
select department_id,min(salary)
from employees
group by department_id
having min(salary)>(
select min(salary) from employees where department_id = 50
);
###列子查询
多行操作符:
#案例1:返回location_id是1400或1700的部门中的所有员工姓名
select
last_name,
department_id
from
employees e
where
department_id in (
select
department_id
from
departments
where
location_id in (1400, 1700)
);
#案例2:返回其它部门中比job_id为‘it_prog’部门任一工资低的员工的员 工号、姓名、job_id 以及salary
select
employee_id,
last_name,
job_id,
salary
from
employees e
where
salary < any (
select distinct
salary
from
employees
where
job_id = 'it_prog'
)
and job_id <> 'it_prog';
-------------------两种方式等价------------------------
select
employee_id,
last_name,
job_id,
salary
from
employees e
where
salary < (
select
max(distinct salary)
from
employees
where
job_id = 'it_prog'
)
and job_id <> 'it_prog';
#案例3:返回其它部门中比job_id为‘it_prog’部门所有工资都低的员工 的员工号、姓名、job_id 以及salary
select
employee_id,
last_name,
job_id,
salary
from
employees e
where
salary < all (
select distinct
salary
from
employees
where
job_id = 'it_prog'
)
and job_id <> 'it_prog';
-------------------两种方式等价------------------------
select
employee_id,
last_name,
job_id,
salary
from
employees e
where
salary < (
select
min(distinct salary)
from
employees
where
job_id = 'it_prog'
)
and job_id <> 'it_prog';
注意:因为列子查询中的all和any|some通能能被max或min分组查询后的标量子查询替换,因此用的较少
#行子查询(结果集一行多列或多行多列)
#案例:查询员工编号最小并且工资最高的员工信息
select
*
from
employees
where
employee_id = (
select
min(employee_id)
from
employees
)
and salary = (
select
max(salary)
from
employees
);#用标量子查询写的方式
-------------------两种方式等价------------------------
select
*
from
employees
where
(employee_id, salary) = (
select
min(employee_id),
max(salary)
from
employees
);#用行子查询写得方式
###select 后面
注意:仅仅支持标量子查询
#案例:查询每个部门的员工个数
select
d.*, (
select
count(*)
from
employees e
where
e.department_id = d.department_id
)
from
departments d;
#案例2:查询员工号=102的部门名
select
(
select
department_name
from
departments d
inner join employees e on e.department_id = d.department_id
where
e.employee_id = 102
);
###from后面
注意:将子查询结果充当一个表,要求必须取别名
#案例:查询每个部门的平均工资的工资等级
select
ag_dep.*, g.grade_level
from
(
select
avg(salary) ag,
department_id
from
employees
group by
department_id
) ag_dep
inner join job_grades g on ag_dep.ag between lowest_sal
and highest_sal;
###exists后面(相关子查询)
语法:exists(完整的查询语句)
结果:1或0
select exists(select employee_id from employees);->1
select exists(select employee_id from employees where salary = 30000);->0
存在结果就返回1,不存在就返回0
#案例:查询有员工的部门名
select department_name
from departments d
where exists(
select *
from employees e
where d.department_id = e.department_id
);
原文:https://www.cnblogs.com/jane315/p/12846550.html
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