第四章-连接

内部连接

• 语法结构

  SELECT <select list>
  FROM <first_table>
  <join_type> <second_table>
    [ON <join_condition>]

• 使用范例

  SELECT Person.BusinessEntity.*
    FROM Person.BusinessEntity
    INNER JOIN HumanResources.Employee
        ON Person.BusinessEntity.BusinessEntityID = 
        HumanResources.Employee.BusinessEntityID;

• 内连接具有排他特性.

• INNER JOIN类似于WHERE

外连接

• 外连接的简易语法

  SELECT <SELECT list>
    FROM <the table you want to be the "LEFT" table>
    <LEFT|RIGHT> [OUTER] JOIN <table you want to be the "RIGHT" table>
    ON <join condition>

• 外连接本质上是包含的.

• 假设要知道具体折扣信息, 每一种折扣的数量和哪些商品打折.

  SELECT sso.SpecialOfferID, Description, DiscountPct, ProductID
    FROM Sales.SpecialOffer sso
    LEFT JOIN Sales.SpecialOfferProduct ssop
        ON sso.SpecialOfferID = ssop.SpecialOfferID
    WHERE sso.SpecialOfferID != 1;

• LEFT JOIN保留的是左侧表的字段, 如果右侧表中不存在与左侧表匹配的字段, SQL Server会为其他任意值填充NULL.

• RIGHT JOIN保留的是右侧表的字段, 如果左侧表中不存在与右侧表匹配的字段, SQL Server会为其他任意值填充NULL.

  SELECT sso.SpecialOfferID, Description, DiscountPct, ProductID
    FROM Sales.SpecialOffer sso
    LEFT JOIN Sales.SpecialOfferProduct ssop
        ON sso.SpecialOfferID = ssop.SpecialOfferID
    WHERE sso.SpecialOfferID != 1;
  
  SELECT sso.SpecialOfferID, Description, DiscountPct, ProductID
    FROM Sales.SpecialOfferProduct ssop
    RIGHT JOIN Sales.SpecialOffer sso
        ON ssop.SpecialOfferID = sso.SpecialOfferID
    WHERE sso.SpecialOfferID != 1;

• 返回与任何产品不关联的折扣名称

  SELECT Description
    FROM Sales.SpecialOffer sso
    LEFT JOIN Sales.SpecialOfferProduct ssop
        ON sso.SpecialOfferID = ssop.SpecialOfferID
    WHERE sso.SpecialOfferID != 1
    AND ssop.SpecialOfferID IS NULL;
  
  SELECT Description
    FROM Sales.SpecialOfferProduct ssop
    RIGHT JOIN Sales.SpecialOffer sso
        ON ssop.SpecialOfferID = sso.SpecialOfferID
    WHERE sso.SpecialOfferID != 1
    AND ssop.SpecialOfferID IS NULL;

• NULL值不等于NULL值.

  IF (NULL = NULL)
    PRINT ‘It Does‘
  ELSE
    PRINT ‘It Doesn‘‘t‘

处理更复杂的外部连接

-- 返回所有供应商的地址
SELECT v.VendorName, a.Address
    FROM Vendors v
    LEFT JOIN VendorAddress va
        ON v.VendorID = va.VendorID
    LEFT JOIN Address a
        ON va.AddressID = a.AddressID;

SELECT v.VendorName, a.Address
    FROM VendorAddress va
    JOIN Address a
        ON va.AddressID = a.AddressID
    RIGHT JOIN Vendors v
        ON v.VendorID = va.VendorID;

-- 使用分组连接
SELECT v.VendorName, a.Address
    FROM Vendors v
    LEFT JOIN (
        VendorAddress va
        JOIN Address a
        ON va.AddressID = a.AddressID
    )
    ON v.VendorID = va.VendorID;

完全连接

• FULL JOIN将左右两侧的数据全部匹配, 并返回所有的记录. 无论记录在JOIN的哪一侧.

• FULL JOIN查询出来是两个表的并集.

  SELECT a.Address, va.AddressID, v.VendorID, v.VendorName
    FROM VendorAddress va
    FULL JOIN Address a
        ON va.AddressID = a.AddressID
    FULL JOIN Vendors v
        ON va.VendorID = v.VendorID;

交叉连接

• 交叉连接是将JOIN左侧的所有记录与另一侧的所有记录连接.

• 交叉连接返回的是JOIN两侧表的所有记录的笛卡尔积.

• 交叉连接使用范例

  SELECT v.VendorName, a.Address
    FROM Vendors v
    CROSS JOIN Address a;

联合

• JOIN 将信息水平连接(添加更多列)

• UNION 将数据垂直连接(添加更多行)

• UNOIN查询的关键点

  • 所有联合的查询必须在SELECT列表中有相同的列数.
  • UNION返回的结果集的标题(列名)仅从第一个查询获得.
  • 查询中的对应列的数据类型必须隐式一致.
  • UNION查询的默认返回选项为DISTINCT, 而不是ALL.

• UNION使用范例:

  SELECT FirstName + ‘ ‘ + LastName AS Name
    FROM Person.Person pp
    JOIN Person.EmailAddress pe
        ON pp.BusinessEntityID = pe.BusinessEntityID
    JOIN Sales.Customer sc
        ON pp.BusinessEntityID = sc.CustomerID
  
    UNION
  SELECT FirstName + ‘ ‘ + LastName AS Name
    FROM Person.Person pp
    JOIN Person.EmailAddress pe
        ON pp.BusinessEntityID = pe.BusinessEntityID
    JOIN Purchasing.Vendor pv
        ON pp.BusinessEntityID = pv.BusinessEntityID

• UNION去除重复行, SQL Server会将具有相等NULL列的行视为重复行.

  -- 查询添加特价供应产品的名称
  SELECT P.ProductNumber, ‘Less than 100 left‘ AS SpecialOffer
    FROM Production.Product P
    JOIN Production.ProductInventory I
        ON I.ProductID = P.ProductID
    WHERE I.Quantity < 100
    UNION
  SELECT P.ProductNumber, SO.Description
    FROM Production.Product P
    JOIN Sales.SpecialOfferProduct O
        ON P.ProductID = O.ProductID
    JOIN Sales.SpecialOffer SO
        ON SO.SpecialOfferID = O.SpecialOfferID
    WHERE O.SpecialOfferID > 1

小结

• 要排除不匹配的字段的时候 使用内部链接(INNER JOIN | JOIN)
• 要检索尽可能匹配的数据, 但又要包含JOIN一侧的表的所有数据, 使用外部链接.(LEFT | RIGHT OUTER JOIN)
• 要检索尽可能匹配的数据, 但又要包含JOIN两侧的表的所有数据, 使用完全外连接.(FULL OUTER JOIN)
• 要基于两个表的记录建立笛卡尔积时, 使用交叉连接.(CROSS JOIN)
• 要将第二个查询结果附加到第一个查询结果时, 使用联合.(UNION | UNION ALL)

练习题

• 针对AdventureWorks数据库编写一条查询语句, 要求返回Name列并包含NationalIDNumber为112457891的雇员的姓.

  SELECT P.LastName AS Name
    FROM Person.Person P
    JOIN HumanResources.Employee E
        ON P.BusinessEntityID = E.BusinessEntityID
    WHERE E.NationalIDNumber = ‘112457891‘;

• 针对AdventureWorks数据库编写一条查询语句, 要求返回所有产品的ID和Name列, 包括没有特价供应的所有产品和有No Discount特价供应的所有产品.

  SELECT P.ProductID, P.Name
    FROM Production.Product P
    LEFT JOIN Sales.SpecialOfferProduct SSP
        ON P.ProductID = SSP.ProductID
    WHERE SSP.SpecialOfferID IS NULL
    UNION
  SELECT P.ProductID, P.Name
    FROM Production.Product P
    JOIN Sales.SpecialOfferProduct SSP
        ON P.ProductID = SSP.ProductID
    JOIN Sales.SpecialOffer SSO
        ON SSO.SpecialOfferID = SSP.SpecialOfferID
    WHERE SSO.Description = ‘No Discount‘;

SQL Server学习笔记-连接

原文：https://www.cnblogs.com/devinkin/p/10048889.html