SQL_Server的前情介绍
😁大家好呀，今天是我第三次写sql_server，也是最近才学习sql_server，也想着记录一下自己的学习过程，并且分享给大家尼！
一、sql_server技术介绍
SQL Server 是由微软公司（Microsoft）开发的关系型数（RDBMS）。RDBMS 是 SQL 以及所有现代数据库系统的基础，比如 MS SQL Server，IBM DB2，Oracle，MySQL 以及微软的 Microsoft Access。
二、学习前的准备工作
编程软件：SQL Server Management Studio 2012
带好你的小板凳，我们一起扬帆起航！
SQL_Server之多表查询
笛卡尔乘积的讲解
在数据库中有一种叫笛卡尔乘积其语法如下：
select * from People,Department此查询结果会将People表的所有数据和Department表的所有数据进行依次排列组合形成新的记录。例如People表有10条记录，Department表有3条记录，则排列组合之后查询结果会有10*3=30条记录.
多表查询
接下来我们来看几个例子吧！
1.查询员工信息，显示部门信息
select * from People,department where People.DepartmentId = department.DepartmentId 
2.查询员工信息，显示职级名称
select * from People,s_rank where People.RankId = s_rank.RankId
3.查询员工信息，显示部门名称，显示职级名称
select * from People,department,s_rank
where People.departmentId = department.DepartmentId and People.RankId = s_rank.RankId 
内连接查询
在数据库的查询过程中，存在有内连接查询，这个时候，我们就需要用到inner这个关键字，下面我们来看几个例子吧！
1.查询员工信息，显示部门信息
select * from People inner join department on People.departmentId = department.DepartmentId
2.查询员工信息，显示职级名称
select * from People inner join s_rank on People.RankId = s_rank.RankId
3.查询员工信息，显示部门名称，显示职级名称
select * from People inner join department on People.departmentId = department.DepartmentId
inner join s_rank on People.RankId = s_rank.RankId
外连接查询(左外连，右外连，全外连)
1.查询员工信息，显示部门信息(左外连)
select * from People left join department on People.departmentId = department.DepartmentId
2.查询员工信息，显示职级名称(左外接)
select * from People left join s_rank on People.RankId = s_rank.RankId
3.查询员工信息，显示部门名称，显示职级名称(左外连)
select * from People left join department on People.departmentId = department.DepartmentId
inner join s_rank on People.RankId = s_rank.RankId
4.右外连
A left join B = B right join A
select * from People right join department on People.departmentId = department.DepartmentId
全外连查询(无论是否符合关系，都要显示数据)
1.select * from People full join department on People.departmentId = department.DepartmentId
多表查询的主要例子
1.查询出武汉地区所有的员工信息，要求显示部门名称，以及员工的详细资料（显示中文别名）
select PeopleId 员工编号,DepartmentName 部门名称,PeopleName 员工姓名,PeopleSex 员工性别,
PeopleBirth 员工生日,PeoPleSalary 月薪,PeoplePhone 电话,PeopleAddress 地址
from People,department where People.departmentId = department.DepartmentId
2.查询出武汉地区所有员工的信息，要求显示部门名称，职级名称以及员工的详细资料
select PeopleId 员工编号,DepartmentName 部门名称,RankName 职级名称, PeopleName 员工姓名,PeopleSex 员工性别,
PeopleBirth 员工生日,PeoPleSalary 月薪,PeoplePhone 电话,PeopleAddress 地址
from People,department,s_rank where People.departmentId = department.DepartmentId
and People.RankId = s_rank.RankId and PeopleAddress = '武汉'
3.根据部门分组统计员工人数，员工工资总和，平均工资，最高工资和最低工资
select DepartmentName 部门名称, count(*) 员工人数,sum(PeopleSalary) 工资总和,avg(PeopleSalary) 平均工资,max(PeopleSalary) 最高工资,min(PeopleSalary) 最低工资
from People,department
where People.departmentId = department.DepartmentId
group by department.DepartmentId,DepartmentName
4.根据部门分组统计员工人数，员工工资总和，平均工资，最高工资和最低工资平均工资在10000元以下的不参与排序。根据平均工资降序排序
select DepartmentName 部门名称, count(*) 员工人数,sum(PeopleSalary) 工资总和,avg(PeopleSalary) 平均工资,max(PeopleSalary) 最高工资,min(PeopleSalary) 最低工资
from People,department
where People.departmentId = department.DepartmentId
group by department.DepartmentId,DepartmentName
having avg(PeopleSalary) >= 15000
order by avg(PeopleSalary) desc
5.根据部门名称，然后根据职位名称分组，统计员工人数，员工工资总和
平均工资，最高工资，和最低工资
select DepartmentName 部门名称,RankName 职级名称, count(*) 员工人数,sum(PeopleSalary) 工资总和,avg(PeopleSalary) 平均工资,max(PeopleSalary) 最高工资,min(PeopleSalary) 最低工资
from People,department,s_rank
where People.departmentId = department.DepartmentId and People.RankId = s_rank.RankId
group by department.DepartmentId,DepartmentName,s_rank.RankId,s_rank.RankName
如何巩固学习
提示：在学习的过程中，我们需要先自行进行思考，而不是一遇到不会的就放弃思考直接看答案，如果最后遇到真的不会的题目，我们可以适当的进行观看答案，看自己的思路是否正确，在作出正确的判断
总结
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