查询数据库中的多个表

Question

我有一个包含这些表和列的数据库：

人

firstname
lastname 
peid

买家

peid

优惠

peid  
pno

我正在尝试选择具有相同pno和不同peid的名称（买方1和买方2）。

FIRSTNAME    LASTNAME   PEID     PNO
-------------------------------------------
Robert       Young      0        18
Stephen      Davison    2        32
Tony         Nguyen     16       28
Lily         Roy        32       14
Aaron        Naidoo     50       51
Adam         Jordan     64       32
Arun         Isaacson   68       27
Charles      Murphy     84       23
Adam         Peter      94       27

我想要的是选择具有相同pno和不同peid的两个买家以及他们共同拥有的pno。例如：

亚当乔丹和斯蒂芬戴维森有32个共同点。 Adam Peter和Arun Isaacson有27个共同点。我正试图选择它们。 上表是我的一个查询的结果，而不是原始表。

---

来自评论：

  

这是我到目前为止所做的事情

select A.firstname || ' ' || A.lastname as Buyer1,
       B.firstname || ' ' || B.lastname as Buyer2
  from person A, person B, buyer One, buyer Two
 where A.peid = One.peid and B.peid = Two.peid
   and (select off.pno  from offer off where off.peid = One.peid) =
       (select off.pno  from offer off where off.peid = Two.peid)
   and (select off.peid from offer off where off.peid = One.peid) <>
       (select off.peid from offer off where off.peid = Two.peid)
 group by A.firstname || ' ' || A.lastname, B.firstname || ' ' || B.lastname;

Answer 1

如果您的DBMS支持CTE（公用表表达式，也就是WITH子句），您可以比不支持CTE更加整洁。但是，许多DBMS不支持它，所以这里是没有CTE的版本。分阶段开发（和测试）答案通常是最容易的。目前还不完全清楚为什么你需要买方表。但是，我们认为它至关重要。

我们可以使用以下查询生成人员列表和他们购买的内容：

SELECT P.*, O.pno
  FROM Person AS P
  JOIN Buyer  AS B ON P.peid = B.peid
  JOIN Offer  AS O ON B.peid = O.peid;

两个回答主要查询，我们需要这个结果集的两个副本，在pno列上加入，条件是peid值不同：

SELECT P1.Name, P1.Peid, P2.Name, P2.Peid, P1.Pno
  FROM (SELECT (P.FirstName || ' ' || P.LastName) AS Name, P.Peid, O.Pno
          FROM Person AS P
          JOIN Buyer  AS B ON P.Peid = B.Peid
          JOIN Offer  AS O ON B.Peid = O.Peid
       ) AS P1
  JOIN (SELECT (P.FirstName || ' ' || P.LastName) AS Name, P.Peid, O.Pno
          FROM Person AS P
          JOIN Buyer  AS B ON P.Peid = B.Peid
          JOIN Offer  AS O ON B.Peid = O.Peid
       ) AS P2
    ON P1.Pno = P2.Pno AND P1.Peid < P2.Peid;

在最后一行使用<的技巧是常见的。它确保您拥有这样的对称查询，然后每对只列出一次。也就是说，如果Person1和Person2都购买了相同的商品，那么Person2和Person1也是如此。但是，<条件确保只显示两行中的一行。它还确保Person1和Person1行以及Person2和Person2行都不会出现。

如果您的DBMS支持CTE，您可以将子查询转换为命名CTE，然后在主查询中引用它。

近似语法：

WITH Purchases AS 
    (SELECT (P.FirstName || ' ' || P.LastName) AS Name, P.Peid, O.Pno
       FROM Person AS P
       JOIN Buyer  AS B ON P.Peid = B.Peid
       JOIN Offer  AS O ON B.Peid = O.Peid
    )
SELECT P1.Name, P1.Peid, P2.Name, P2.Peid, P1.Pno
  FROM Purchases AS P1
  JOIN Purchases AS P2
    ON P1.Pno = P2.Pno AND P1.Peid < P2.Peid;