SELECT 的 CASE WHEN 中的相关子查询如何访问外部表别名？

Question

我使用的是 Oracle 12。我设计了一个简单的示例来尝试传达我面临的问题，因为实际查询过于复杂，无法在此处展示。这个简单的示例包含与复杂查询相同的条件和逻辑。我希望找到一种方法，让位于主 SELECT 内的 WHEN 中的子查询访问从与主 SELECT 处于同一级别的内部连接定义的表别名。我试过这个版本，但它失败了'o'“表或视图不存在”：

 SELECT emp.employeeid, emp.firstname
 CASE 
 WHEN (SELECT count(*) FROM o WHERE o.shipperid <= 15) > 20 THEN 'Yes'
 WHEN (SELECT count(*) FROM o WHERE o.shipperid <= 15) > 10 THEN 'Almost'
 ELSE 'No'
 END AS "Quota Met"
 FROM Employee emp
 INNER JOIN 
 (SELECT employeeid, shipperid FROM Orders 
  WHERE orderdate > sysdate - 30) o
  ON o.employeeid = emp.id
 WHERE emp.zipcode = 22151;

无法让上面的 INNER JOIN 版本正常工作，到目前为止，我最好的解决方法是使用如下所示的 WITH 功能定义 SELECT，但在实际查询中， WITH 块需要几分钟才能完成，因为没有办法限制它与 Employee.employeeid 匹配，就像上面的 INNER JOIN 版本尝试一样。

 WITH MyOrders AS
 (SELECT employeeid, shipperid FROM Orders 
  WHERE orderdate > sysdate - 30
 )
 SELECT emp.id, emp.name, o
  CASE 
   WHEN (SELECT COUNT(*) FROM MyOrders o WHERE o.shipperid <= 15) > 20 THEN 'Yes'
   WHEN (SELECT COUNT(*) FROM MyOrders o WHERE o.shipperid <= 15) > 10 THEN 'Almost'
  ELSE 'No'
  END AS "Quota Met"
 FROM Employee emp
 WHERE emp.zipcode = 22151;

WITH 版本真的是最好的解决方案吗？有什么办法可以重写第一个INNER JOIN版本，让CASE WHEN中的子查询可以访问连接的表别名？

Answer 1

你并不真的需要子查询；如果您将o.shipperid <= 15 移动到内联视图中，那么您可以只计算连接中的匹配项。我已将您的内部联接更改为外部联接，因此您会在零计数时看到“否” - 否则将不匹配，并且根本不会显示该员工。

SELECT emp.employeeid, emp.firstname,
   CASE 
     WHEN count(o.shipperid) > 20 THEN 'Yes'
     WHEN count(o.shipperid) > 10 THEN 'Almost'
     ELSE 'No'
   END AS "Quota Met"
FROM Employee emp
LEFT JOIN (
  SELECT employeeid, shipperid
  FROM Orders 
  WHERE orderdate > sysdate - 30
  AND o.shipperid <= 15
) o
ON o.employeeid = emp.id
WHERE emp.zipcode = 22151
GROUP BY emp.employeeid, emp.firstname;

由于这是聚合，您也需要 group by 子句；对于您真正的、更复杂的查询，这可能会更成问题。

如果您无法在更复杂的查询中移动该条件，因为您使用来自 o 的其他数据不满足，那么您可以使用条件聚合和更多的 case 表达式：

SELECT emp.employeeid, emp.firstname,
   CASE 
     WHEN count(CASE WHEN o.shipperid <= 15 THEN o.shipperid END) > 20 THEN 'Yes'
     WHEN count(CASE WHEN o.shipperid <= 15 THEN o.shipperid END) > 10 THEN 'Almost'
     ELSE 'No'
   END AS "Quota Met"
FROM Employee emp
LEFT JOIN (
  SELECT employeeid, shipperid
  FROM Orders 
  WHERE orderdate > sysdate - 30
) o
ON o.employeeid = emp.id
WHERE emp.zipcode = 22151
GROUP BY emp.employeeid, emp.firstname;

你可能仍然希望它是一个外部连接，但如果不把它改回一个内部连接。

Answer 2

你不能以表的形式访问外部子查询，你必须重复代码

SELECT emp.employeeid
, emp.firstname
, CASE 
  WHEN (
    SELECT count(*) 
    FROM  (
    SELECT employeeid, shipperid 
    FROM Orders 
    WHERE orderdate > sysdate - 30
    ) o 
    WHERE o.shipperid <= 15
  ) > 10 THEN 'Almost'
WHEN (
  SELECT count(*) 
  FROM  (
    SELECT employeeid, shipperid 
    FROM Orders 
    WHERE orderdate > sysdate - 30
    ) o
    WHERE o.shipperid <= 15) > 20 THEN 'Yes'
ELSE 'No' END AS "Quota Met"
FROM Em  ployee emp
INNER JOIN  (
SELECT employeeid, shipperid 
FROM Orders 
WHERE orderdate > sysdate - 30

) o ON o.employeeid = emp.id
WHERE emp.zipcode = 22151;

如果不想重复代码，你可以使用视图

create my_view as  
SELECT employeeid, shipperid 
  FROM Orders 
  WHERE orderdate > sysdate - 30


SELECT emp.employeeid
, emp.firstname
, CASE 
  WHEN (
    SELECT count(*) 
    FROM my_view o
    WHERE o.shipperid <= 15
  ) > 10 THEN 'Almost'
WHEN (
  SELECT count(*) 
  FROM  my_view o
    WHERE o.shipperid <= 15) > 20 THEN 'Yes'
ELSE 'No' END AS "Quota Met"
FROM Em  ployee emp
INNER JOIN  my_viewq o ON o.employeeid = emp.id
WHERE emp.zipcode = 22151;