sql server 简单语句整合

1.去重distinct ， group by

select distinct userid,username from 表名

select userid,username from 表名 group by userid,username

2.去空格 replace  ltrim rtrim

Replace(name,' ','')

select ltrim('  test  ')        --去除左边的空格  
select rtrim('  test  ')        --去除右边的空格  
select ltrim(rtrim('  test  ')) --去除首尾空格  

3.表表关联语句  where条件关联、左/右关联

--用select更出要显示的字段
select b.ID,GID,GName,GStand,GInPrice,GOutPrice,GBaseClass,b_Date,b_OPerson,b_Time
--来自给tbl_Goods表和tbl_SellBill表，给tbl_Goods表设置一个别名a,给tbl_SellBill表设置一个别名b
from tbl_Goods a,tbl_SellBill b
--两个表关联的条件是 a.id=b.gid
where a.id=b.gid

select * from tableA a left  join  tableB b on a.id = b.id
select * from tableA a right join  tableB b on a.id = b.id
select * from tableA a inner join  tableB b on a.id = b.id
left join(左联接) 返回包括左表中的所有记录和右表中联结字段相等的记录 
right join(右联接) 返回包括右表中的所有记录和左表中联结字段相等的记录
inner join(等值连接) 只返回两个表中联结字段相等的行

4.求比计算机系中某一学生年龄小的其他系的学生（any）

方法一：
select * 
from student 
where
    sdept!="CS" AND  
    sage < ANY (select Sage from Student where Sdept="CS");
    
方法二：
select  *
from Student
where   
   Sdept!=’CS’ AND  
   Sage < (select MAX(Sage) from Student where Sdept="CS");

5.多重句子

Teacher(tno, tname, salary, dno)
Department(dno, dname)

查询语句：
select Tname，Salary
from Teacher
where 
    Dno = "D01" AND 
    salary IN(
        select salary from  teacher
        where Dno =（select DNO from department where  Dname="国贸")
    );

6.带有exists的相关子查询

不相关子查询：    
select Sname from student where sno IN ( select sno from SC where Cno = "C1" );
 
相关子查询
select Sname from student
where exists (select * from SC where student.sno=SC.sno AND Cno = "C1" );

 7.交，并，差　

1 Union 取合集并过滤重复数据

--1 Union 取合集并过滤重复数据  
--结果显示: 1,2,3,4,5  
SELECT * FROM A  
UNION     
SELECT * FROM B;  

2 Union all 取合集不过滤重复数据

--2 Union all 取合集不过滤重复数据  
--结果显示:1,2,3,4,1,2,5  
SELECT * FROM A  
UNION  all  
SELECT * FROM B;  

3 Intersect 取交集(两个表中都有数据)

--3 Intersect 取交集  
--结果显示:1,2  
SELECT * FROM A  
Intersect    
SELECT * FROM B;  

4 except 取差集(取A-B的记录)

--4 except 取差集  
--结果显示:3,4  
SELECT * FROM A  
except    
SELECT * FROM B;