SQL Server 2012 - 多表鏈接查詢

-- 交叉鏈接產生笛卡爾值 (X*Y)
SELECT  * 
FROM    Student
        cross Join dbo.ClassInfo  
--另一種寫法
SELECT  * 
FROM    Student , ClassInfo  

-- 內鏈接 (Inner 能夠省略)
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，一般是 主外鍵，可是不限於主外鍵
	  -- on 條件，容許有多個，並且多是針對某個表的
SELECT  *
FROM    Student
        INNER  JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID
		AND dbo.Student.Class IN (1,2)  -- 針對Student表增長查詢條件

-- 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;

--自鏈接  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;
-- 全外鏈接
SELECT  *
FROM    Student
        FULL OUTER  JOIN dbo.ClassInfo ON dbo.Student.Class = dbo.ClassInfo.ID;

-- 外鏈接和內鏈接的區別是：
-- 內鏈接，on條件不符合的話，會過濾掉數據
-- 外鏈接，以保留表爲主，on條件成立顯示數據，不然顯示NULL

--Union 縱向鏈接
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 去重複
SELECT  StuSex
FROM    Student WHERE StuID<=2
UNION
SELECT StuSex
FROM dbo.Student WHERE StuID>2

--Union 顯示所有
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，能夠給默認的值，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爲準,以下面學生身高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能夠相互轉換,以下 StuID和Height之間能夠轉換，能夠縱向鏈接
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 寫在最後
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