SQL Server 2012 - 多表連接查詢

-- 交叉連接產生笛卡爾值 (X*Y)
SELECT  * 
FROM    Student
        cross Join dbo.ClassInfo  
--另外一種寫法
SELECT  * 
FROM    Student , ClassInfo  

-- 內連接 (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 條件，通常是 主外鍵，但是不限於主外鍵
	  -- on 條件，允許有多個，而且可能是針對某個表的
SELECT  *
FROM    Student
        INNER  JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID
		AND dbo.Student.Class IN (1,2)  -- 針對Student表增加查詢條件

-- 不等於 (笛卡爾值減去 相等的值)
SELECT  *
FROM    Student
        INNER  JOIN dbo.ClassInfo ON dbo.Student.Class <> dbo.ClassInfo.ID;

--自連接  從Class表中查詢到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;
-- 全外連接
SELECT  *
FROM    Student
        FULL OUTER  JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID;

-- 外連接和內連接的區別是：
-- 內連接，on條件不符合的話，會過濾掉數據
-- 外連接，以保留表為主，on條件成立顯示數據，否則顯示NULL

--Union 縱向連接
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 去重復
SELECT  StuSex
FROM    Student WHERE StuID<=2
UNION
SELECT StuSex
FROM dbo.Student WHERE StuID>2

--Union 顯示全部
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

　　Union縱向查詢的幾個說明：

--Union 
--1,列的數目必須相同，對於沒有的列，可以給默認的值，如NULL
SELECT StuID,StuName,StuSex,Class,Height FROM dbo.Student WHERE StuID<2
UNION 
SELECT  StuID,StuName,StuSex,Class,180 AS Height  FROM dbo.Student WHERE StuID=2
UNION 
SELECT  StuID,StuName,StuSex,Class,NULL  FROM dbo.Student WHERE StuID>2

--2,查詢結果呈現的列名稱是以第一列為准,如下面學生身高StuHeight
SELECT StuID,StuName,StuSex,Class,Height AS StuHeight FROM dbo.Student WHERE StuID<2
UNION 
SELECT  StuID,StuName,StuSex,Class,180 AS Height  FROM dbo.Student WHERE StuID=2
UNION 
SELECT  StuID,StuName,StuSex,Class,NULL AS hei FROM dbo.Student WHERE StuID>2

--3, 縱向對應的列的數據類型之間可以相互轉換,如下 StuID和Height之間可以轉換，可以縱向連接
SELECT StuID,StuName,StuSex,Class,Height AS StuHeight FROM dbo.Student WHERE StuID<2
UNION 
SELECT  Height,StuName,StuSex,Class,NULL AS hei FROM dbo.Student WHERE StuID>2

--4，對Union的查詢結果進行排序，Order 寫在最后
SELECT StuID,StuName,StuSex,Class,Height AS StuHeight FROM dbo.Student WHERE StuID<2
UNION 
SELECT  StuID,StuName,StuSex,Class,180 AS Height  FROM dbo.Student WHERE StuID=2
UNION 
SELECT  StuID,StuName,StuSex,Class,NULL AS hei FROM dbo.Student WHERE StuID>2
ORDER BY Class