Question

我正在使用adventureworks2012：

第一个查询返回43行并且没有空值，而第二个返回超过19,000个并且返回大量空值。

我认为外部联接返回了查询左侧的行，即使条件不满足，因此两个查询应该是等价的？

- 1

SELECT c.CustomerID, s.SalesOrderID, s.OrderDate
FROM Sales.Customer AS c
LEFT OUTER JOIN Sales.SalesOrderHeader AS s ON c.CustomerID = s.CustomerID
WHERE s.OrderDate = '2005/07/01';

- 2

WITH orders AS (
SELECT SalesOrderID, CustomerID, OrderDate
FROM Sales.SalesOrderHeader
WHERE OrderDate = '2005/07/01'
)
SELECT c.CustomerID, orders.SalesOrderID, orders.OrderDate
FROM Sales.Customer AS c
LEFT OUTER JOIN orders ON c.CustomerID = orders.CustomerID
ORDER BY orders.OrderDate DESC;

Answer 1

这是因为查询1中的WHERE子句。WHERE子句过滤了要连接的两个表的最终结果。您需要将条件从WHERE子句移动到ON，首先过滤记录（在表Sales.SalesOrderHeader上），然后再将其连接到另一个表（Sales.Customer） ，you can also find it here.

SELECT c.CustomerID, s.SalesOrderID, s.OrderDate
FROM   Sales.Customer AS c
       LEFT OUTER JOIN Sales.SalesOrderHeader AS s 
           ON c.CustomerID = s.CustomerID  AND 
              s.OrderDate = '2005/07/01';

Answer 2

您通过引用位于其中的表来杀死第一个外部只需将该条件移入连接

即可

SELECT c.CustomerID, s.SalesOrderID, s.OrderDate
 FROM Sales.Customer AS c
 LEFT OUTER JOIN Sales.SalesOrderHeader AS s 
   ON c.CustomerID = s.CustomerID
  AND s.OrderDate = '2005/07/01';

为什么这两个查询返回不同的行数

2 个答案: