Question

我有以下代码

SELECT * 
FROM customer
INNER JOIN
    (SELECT   
         customerid, newspapername, enddate, n.publishedby 
     FROM
         newspapersubscription ns, newspaper n 
     WHERE 
         publishedby IN (SELECT publishedby 
                         FROM newspaper 
                         WHERE ns.newspapername = n.NewspaperName)
UNION

SELECT
    customerid, Magazinename, enddate, m.publishedby 
FROM
    magazinesubscription ms, magazine m 
WHERE
    publishedby IN (SELECT publishedby 
                    FROM magazine 
                    WHERE ms.Magazinename = m.MagazineName)) ON customer.customerid = customerid
ORDER BY 
    customer.customerid;

customer表具有以下内容：

 customerid | customername | customersaddress

此查询返回以下结果：

customerid | customername | customersaddress | customerid | newspapername | enddate| publishedby

我真正想要的是

customerid | customername | customersaddress | newspapername | magazinename | enddate| publishedby

此处，如果存在magazinename，则newspapername字段应为空白，反之亦然。此外，不应出现来自联合操作的customerid的重复字段，而在我的结果中，newspapername和magazinename的值都放在newspapername标题下。

我该怎么做？

Answer 1

由于您使用'*'查询表，因此您将始终获得两个表中的所有列。要省略此列，您必须手动命名要查询的所有列。要满足您的其他需求，您只需在联合查询中的每个子句中插入一个虚拟列。下面的示例应该可以满足您的需求 -

SELECT customer.customerid, customer.customername, customer.customeraddress, newspapername, magazinename, enddate, publishedby 
FROM customer
INNER JOIN
(select  customerid, newspapername, null Magazinename, enddate, n.publishedby 
 from newspapersubscription ns, newspaper n 
 where publishedby in(select publishedby 
                    from newspaper 
                    where ns.newspapername = n.NewspaperName)
UNION
select  customerid, null newspapername, Magazinename, enddate, m.publishedby 
from magazinesubscription ms, magazine m 
 where publishedby in(select publishedby 
                    from magazine 
                     where ms.Magazinename = m.MagazineName))
on customer.customerid = customerid
ORDER BY customer.customerid;

Answer 2

要获得所需的投影，请构建正确形状的子查询，并使用UNION来获取结果集。 UNION ALL比UNION更好，因为它避免了一种排序：你知道你会得到一个独特的集，因为你正在加入两个不同的表。

select * from (
    select customer.* 
           , n.newspapername
           , null as magazinename
           , ns.enddate
          , n.publishedby 
    from customer
        join newspapersubscription ns 
            on ns.customerid = customer.customerid
        join newspaper n
            on  n.newspapername = ns.newspapername 
    union all
    select customer.* 
           , null as newspapername
           , m.magazinename
           , ms.enddate
           , m.publishedby 
    from customer
        join magazinesubscription  ms 
            on ms.customerid = customer.customerid
        join magazine m
            on  m.magazinename = ms.magazinename 
            )
order by customerid, newspapername nulls last, magazinename ;

以下是我的玩具数据集的输出（缺少publishedby列：

CUSTOMERID CUSTOMERNAME         NEWSPAPERNAME          MAGAZINENAME           ENDDATE
---------- -------------------- ---------------------- ---------------------- ---------
        10 DAISY-HEAD MAISIE    THE DAILY BUGLE                               30-SEP-17
        30 FOX-IN-SOCKS         THE DAILY BUGLE                               30-SEP-17
        30 FOX-IN-SOCKS         THE WHOVILLE TIMES                            30-SEP-16
        30 FOX-IN-SOCKS                                GREEN NEWS             31-DEC-17
        30 FOX-IN-SOCKS                                TWEETLE BEETLE MONTHLY 31-DEC-16
        40 THE LORAX                                   GREEN NEWS             31-DEC-18

6 rows selected.

SQL>

如何从SQL中的连接中删除重复的列

2 个答案: