第三章.SQL联接

交叉联接

语法

SELECT 
    C.custid,
    E.empid
FROM Sales.Customers AS C
    CROSS JOIN HR.Employees AS E
--ANSI SQL-92语法

SELECT
    C.custid,
    E.empid
FROM Sales.Customers AS C,HR.Employees AS E
--ANSI SQL-89语法
--两者之间没有逻辑或性能差异，但建议使用ANSI SQL-92语法

自交叉联接

SELECT 
    E1.empid,E1.firstname,E1.lastname,
    E2.empid,E2.firstname,E2.lastname
FROM HR.Employees AS E1
    CROSS JOIN HR.Employees AS E2

生成数字表

SELECT 
    1 + D1.digit*100 + D2.digit + D3.digit * 10 AS N
FROM dbo.Digits AS D1
    CROSS JOIN dbo.Digits AS D2
    CROSS JOIN dbo.Digits AS D3

内部联接

--ANSI SQL-92
SELECT
    E.empid,E.firstname,E.lastname,
    O.orderid
FROM HR.Employees AS E
JOIN Sales.Orders AS O
ON E.empid = O.empid
--内部联接，应在表名称之间指定INNER JOIN关键词，由于内部联接是默认是内部联接所以INNER可以省略

--ANSI SQL-89
SELECT
    E.empid,E.firstname,E.lastname,
    O.orderid
FROM HR.Employees AS E
JOIN Sales.Orders AS O
WHERE E.empid = O.empid

内部联接安全性

推荐使用ANSI-SQL 92

更多联接示例

复合联接

SELECT
    OD.orderid,OD.productid,OD.qty,
    ODA.dt,ODA.loginname,ODA.oldval,ODA.newval
FROM Sales.OrderDetails AS OD
    JOIN Sales.OrderDetailsAudit AS ODA
    ON OD.orderid = ODA.orderid
    AND OD.productid = ODA.productid
WHERE ODA.columnname = N'qty';

不等联接

--不等联接就是联接条件涉及除等号以外的任何运算符
SELECT
    E1.empid,E1.firstname,E1.lastname,
    E2.empid,E2.firstname,E2.lastname
FROM HR.Employees AS E1
    JOIN HR.Employees AS E2
    ON E1.empid < E2.empid

多联接查询

--下面查询联接Customer和Orders表以匹配客户的订单
--然后以第一个联接的结果和OrderDetails表以匹配订单的订单行
SELECT
    C.custid,C.companyname,O.orderid,
    OD.productid,OD.qty
FROM Sales.Customers AS C
    JOIN Sales.Orders AS O
    ON C.custid = O.custid
    JOIN Sales.OrderDetails AS OD
    ON O.orderid = OD.orderid

外部联接

LEFT OUTER JOIN在两张表进行连接查询时，会返回左表所有的行，即使右表中没有匹配的记录，右表符合ON或者WHERE下的条件。

RIGHT OUTER JOIN在两张表进行连接查询时，会返回右表所有的行，即使左表中没有匹配的记录，左表符合ON或者WHERE下的条件。

FULL OUTER JOIN两张表同时符合ON和WHERE下的条件。

SELECT
    C.custid,C.companyname,
    O.custid
FROM Sales.Customers AS C
LEFT OUTER JOIN Sales.Orders AS O
ON C.custid = O.custid

第一阶段：FROM

第二阶段：ON

第三阶段：... OUTER JOIN

第四阶段：WHERE

练习

--3.6.1
SELECT
    empid,firstname,lastname,n
FROM HR.Employees JOIN dbo.Nums 
ON n <= 5

--3.6.3
SELECT
    O.custid,COUNT(DISTINCT OD.orderid) AS numbers,SUM(qty) AS totalqty
FROM Sales.Customers AS C JOIN Sales.Orders AS O 
        ON C.country = 'USA' AND C.custid = O.custid
     JOIN Sales.OrderDetails OD ON O.orderid = OD.orderid
GROUP BY O.custid

--3.6.4
SELECT
    C.custid,companyname,orderid,orderdate
FROM Sales.Customers AS C LEFT JOIN Sales.Orders AS D 
    ON C.custid = D.custid

--3.6.5
SELECT
    C.custid,companyname,orderid,orderdate
FROM Sales.Customers AS C LEFT JOIN Sales.Orders AS D 
    ON C.custid = D.custid
WHERE orderid IS NULL

--3.6.6
SELECT
    *
FROM Sales.Customers AS C JOIN Sales.Orders AS O 
    ON orderdate = '20070212' AND C.custid = O.custid;

--3.6.7
SELECT
    C.custid,companyname,
    CASE WHEN o.orderid IS NULL THEN 'NO'
    ELSE 'YES'
    END AS HasORderOn20070212
FROM Sales.Customers AS C LEFT OUTER JOIN Sales.Orders AS O 
    ON O.custid = C.custid AND orderdate = '20070212';