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SQL query help - have two where conditons in join condition

我有以下表格，如下所示。我已经提到了我需要的东西。我已经在这里SQL query help - have two where conditons in join condition发布了一个链接，我已经尝试了但是无法通过。再次明确地提出我需要的东西：

Book
BookId, BookName

Desk
DeskId, BookId ,DeskName

CounterParty
CPId, CpName

Trade
TradeId, Buyer, Seller

这是买方和卖方数据的方式：

Buyer Seller
B3232  B323
C32    B222
B323   C323

根据这两列中的起始字符B或C，我需要连接Book或CP表来检查ID。

I need **t.TradingDeskName, b.BookName, c.CpName, t.Buyer, t.Seller.**

非常感谢任何帮助。

谢谢，马尼

p.s：我试图通过SQL或Linq到Sql来完成这项工作。

最近的查询，但有更多要修复：

SELECT DISTINCT desk.Name as TradingDeskName, b.Name as Book, t.Seller, t.Buyer, c.PartyName, FROM TradingDesk AS desk 

 RIGHT JOIN Book as b
    ON  b.TradingDeskId = d.Id
 RIGHT JOIN Trade as t  
    ON LEFT(t.Buyer, 1) = 'B' AND SUBSTRING(t.Buyer, 2, len(t.Buyer)) = b.Id
 LEFT JOIN Book as b1 
    ON LEFT(t.Seller, 1) = 'B' AND SUBSTRING(t.Seller, 2, len(t.Seller)) = b1.Id 
 LEFT JOIN CounterParty as c 
    ON LEFT(t.Buyer, 1) = 'C' AND SUBSTRING(t.Buyer, 2, len(t.Buyer)) = c.PartyId 
 LEFT JOIN CounterParty as c1 
    ON LEFT(t.Seller, 1) = 'C' AND SUBSTRING(t.Seller, 2, len(t.Seller)) = c1.PartyId

正如我所提到的，我需要： Desk.Name - B.Name - T.Seller - T.Buyer- C.PartyName

如果T.Seller或T.Buyer值以'C'开头（来自CounterParty表），则C.PartyName将具有该值，否则将为null。

通过上面的查询，我在Desk.Name，B.Name和gettig C.PartyName的逻辑中都有空值。

Answer 1

你可以这样做（未经测试）：

select 
  t.Buyer, 
  t.Seller, 
  case when t.Buyer like 'B%' THEN (select BookName from Book where BookId = t.Buyer)
                              ELSE (select CpName from Counterparty where CPId = t.Buyer)
  end BuyerName,
  case when t.Buyer like 'B%' THEN (select DeskName from Desk where BookId = t.Buyer)
                              ELSE NULL
  end BuyerDeskName,
  case when t.Seller like 'B%' THEN (select BookName from Book where BookId = t.Seller)
                               ELSE (select CpName from Counterparty where CPId = t.Seller)
  end SellerName,
  case when t.Seller like 'B%' THEN (select DeskName from Desk where BookId = t.Seller)
                               ELSE NULL
  end SellerDeskName,
from 
  Trade t

您遇到的问题是，由于您要加入的表是数据驱动的，因此您无法在FROM子句中指定它。

Answer 2

我可以通过几种方式来实现预期的结果但是因为首先应该首先考虑，所以如果可能的话，我建议修改数据库设计。

所以，以下是我可以解决的2个查询：

查询1

SELECT `t`.*,
(CASE 
    WHEN LEFT(`t`.`Buyer`, 1) = 'B' THEN
        (SELECT `b`.`BookName`
         FROM `Book` `b`
         WHERE `b`.`BookId` = SUBSTRING(`t`.`Buyer`, 2))
    ELSE (SELECT `c`.`CPName`
          FROM `CounterParty` `c`
          WHERE `c`.`CPId` = SUBSTRING(`t`.`Buyer`, 2))
END) AS `buyer_name`,
(CASE
    WHEN LEFT(`t`.`Seller`, 1) = 'B' THEN
        (SELECT `b`.`BookName`
         FROM `Book` `b`
         WHERE `b`.`BookId` = SUBSTRING(`t`.`Seller`, 2))
    ELSE (SELECT `c`.`CPName`
          FROM `CounterParty` `c`
          WHERE `c`.`CPId` = SUBSTRING(`t`.`Seller`, 2))
END) AS `seller_name`
FROM `Trade` `t`

查询2

SELECT *
FROM `Trade` `t`
LEFT JOIN `Book` `b` ON LEFT(`t`.`Buyer`, 1) = 'B' AND SUBSTRING(`t`.`Buyer`, 2) = `b`.`BookId`
LEFT JOIN `Book` `b1` ON LEFT(`t`.`Seller`, 1) = 'B' AND SUBSTRING(`t`.`Seller`, 2) = `b1`.`BookId`
LEFT JOIN `CounterParty` `c` ON LEFT(`t`.`Buyer`, 1) = 'C' AND SUBSTRING(`t`.`Buyer`, 2) = `c`.`CPId`
LEFT JOIN `CounterParty` `c1` ON LEFT(`t`.`Seller`, 1) = 'C' AND SUBSTRING(`t`.`Seller`, 2) = `c1`.`CPId`;

上述两个查询都返回相同的结果，但格式不同。请试着看看哪一个最适合你。

此外，从您的问题中不清楚表格Desk在哪里适合以及它与其他表格有什么关系。请随意从Desk添加您需要的相应列。

请注意，建议的查询是在MySQL中。目前还不是很清楚你在运行什么系统 - 你在帖子中提到过你正在尝试使用SQL或Linq SQL，而你在标签中提到的一切都是+ MySQL。

SQL查询帮助 - 基于条件连接多个列

2 个答案: