mysql--多表联查

一般来说，我们为了得到更完整的结果，我们需要从两个或更多的表中获取结果，我一般都是用select xxx,xxx from 表1，表2 where 表1.xxx=表2.xxx,我们一般都是进行的是这般的操作，其实MySQL中还有一种操作，那就是join的操作，例如底下有两个表:

请看 "Persons" 表：

Id_PLastNameFirstNameAddressCity1AdamsJohnOxford StreetLondon2BushGeorgeFifth AvenueNew York3CarterThomasChangan StreetBeijing

接下来请看 "Orders" 表：

Id_OOrderNoId_P177895324467833224561424562153476465
以上的这两个表，然后我们把它们进行连接查询

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1. SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo  
2. FROM Persons, Orders  
3. WHERE Persons.Id_P = Orders.Id_P   

结果集：

LastNameFirstNameOrderNoAdamsJohn22456AdamsJohn24562CarterThomas77895CarterThomas44678

如果使用关键词JOIN来从两个表中获取数据

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNoFROM PersonsINNER JOIN OrdersON Persons.Id_P = Orders.Id_PORDER BY Persons.LastName

结果集：

LastNameFirstNameOrderNoAdamsJohn22456AdamsJohn24562CarterThomas77895CarterThomas44678则inner join 与上面的普通查询没有区别

不同的 SQL JOIN

• JOIN: 如果表中有至少一个匹配，则返回行
• LEFT JOIN: 即使右表中没有匹配，也从左表返回所有的行
• RIGHT JOIN: 即使左表中没有匹配，也从右表返回所有的行
• FULL JOIN: 只要其中一个表中存在匹配，就返回行 

LEFT JOIN 关键字语法

"Persons" 表：

Id_PLastNameFirstNameAddressCity1AdamsJohnOxford StreetLondon2BushGeorgeFifth AvenueNew York3CarterThomasChangan StreetBeijing

"Orders" 表：

Id_OOrderNoId_P177895324467833224561424562153476465

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNoFROM PersonsLEFT JOIN OrdersON Persons.Id_P=Orders.Id_PORDER BY Persons.LastName

结果集：

LastNameFirstNameOrderNoAdamsJohn22456AdamsJohn24562CarterThomas77895CarterThomas44678BushGeorge 

SQL RIGHT JOIN 关键字

"Persons" 表：

Id_PLastNameFirstNameAddressCity1AdamsJohnOxford StreetLondon2BushGeorgeFifth AvenueNew York3CarterThomasChangan StreetBeijing

"Orders" 表：

Id_OOrderNoId_P177895324467833224561424562153476465

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNoFROM PersonsRIGHT JOIN OrdersON Persons.Id_P=Orders.Id_PORDER BY Persons.LastName

结果集：

LastNameFirstNameOrderNoAdamsJohn22456AdamsJohn24562CarterThomas77895CarterThomas44678  34764

SQL FULL JOIN 关键字

"Persons" 表：

Id_PLastNameFirstNameAddressCity1AdamsJohnOxford StreetLondon2BushGeorgeFifth AvenueNew York3CarterThomasChangan StreetBeijing

"Orders" 表：

Id_OOrderNoId_P177895324467833224561424562153476465

全连接（FULL JOIN）实例

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNoFROM PersonsFULL JOIN OrdersON Persons.Id_P=Orders.Id_PORDER BY Persons.LastName

结果集：

LastNameFirstNameOrderNoAdamsJohn22456AdamsJohn24562CarterThomas77895CarterThomas44678BushGeorge   34764