-- 交叉连接产生笛卡尔值 (X*Y)
SELECT *
FROM Student
  cross Join dbo.ClassInfo 
--另外一种写法
SELECT *
FROM Student , ClassInfo 
-- 内连接 (Inner 可以省略)
SELECT *
FROM Student
  JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID;
-- Inner Join
SELECT *
FROM Student
  INNER JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID;
  -- on 条件，通常是 主外键，但是不限于主外键
  -- on 条件，允许有多个，而且可能是针对某个表的
SELECT *
FROM Student
  INNER JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID
  AND dbo.Student.Class IN (1,2) -- 针对Student表增加查询条件
-- 不等于 (笛卡尔值减去 相等的值)
SELECT *
FROM Student
  INNER JOIN dbo.ClassInfo ON dbo.Student.Class <> dbo.ClassInfo.ID;
--自连接 从Class表中查询到Class所在的系
SELECT T1.* ,T2.ClassName FROM dbo.ClassInfo AS T1
  INNER JOIN dbo.ClassInfo AS T2 ON T1.PID=T2.ID
-- Left Join
SELECT *
FROM Student
  Left JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID;
-- Right Join
SELECT *
FROM Student
  RIGHT JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID;
-- 全外连接
SELECT *
FROM Student
  FULL OUTER JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID;
-- 外连接和内连接的区别是：
-- 内连接，on条件不符合的话，会过滤掉数据
-- 外连接，以保留表为主，on条件成立显示数据，否则显示NULL
--Union 纵向连接
SELECT StuID,StuName,StuEnName,StuAge,StuBirthday
FROM Student WHERE StuID<=2
UNION
SELECT StuID,StuName,StuEnName,StuAge,StuBirthday
FROM dbo.Student WHERE StuID>2
--Union 去重复
SELECT StuSex
FROM Student WHERE StuID<=2
UNION
SELECT StuSex
FROM dbo.Student WHERE StuID>2
--Union 显示全部
SELECT StuSex
FROM Student WHERE StuID<=2
UNION ALL
SELECT StuSex
FROM dbo.Student WHERE StuID>2
--Except 差集,排除
SELECT StuID,StuName,StuEnName,StuAge,StuBirthday
FROM Student
Except
SELECT StuID,StuName,StuEnName,StuAge,StuBirthday
FROM dbo.Student WHERE StuID<=2
--Intersect 交集
SELECT StuID,StuName,StuEnName,StuAge,StuBirthday
FROM Student WHERE StuID>=2
Except
SELECT StuID,StuName,StuEnName,StuAge,StuBirthday
FROM dbo.Student WHERE StuID<=3