Question

所以我有3张桌子，

Companies (company_id, company_name)
Stock_Exchanges(stock_exchange_id, stock_exchange_name, stock_exchange_manager)
Stock_Exchanges_Companies(stock_exchange_id, company_id)

我正在尝试选择所有不在某个stock_exchange的公司。

到目前为止，我有这个：

SELECT COMPANY_NAME
  FROM COMPANIES c
  LEFT OUTER JOIN STOCK_EXCHANGES_COMPANIES sec 
    ON c.COMPANY_ID = sec.COMPANY_ID
  LEFT OUTER JOIN STOCK_EXCHANGES se 
    ON sec.STOCK_EXCHANGE_ID = se.STOCK_EXCHANGE_ID
 WHERE sec.COMPANY_ID != se.COMPANY_ID;

当我使用WHERE子句时，它返回零行。任何帮助将不胜感激！

Answer 1

嘿伙计们我设法解决了这个问题！

我添加了一个额外的AND子句：

SELECT COMPANY_NAME
FROM COMPANIES c
LEFT OUTER JOIN STOCK_EXCHANGES_COMPANIES sec ON c.COMPANY_ID = sec.COMPANY_ID
LEFT OUTER JOIN STOCK_EXCHANGES se ON sec.STOCK_EXCHANGE_ID = se.STOCK_EXCHANGE_ID
WHERE sec.COMPANY_ID != se.COMPANY_ID
    AND sec.stock_exchange_id != idhere;

Answer 2

您根据匹配的stockExchangeID加入表，然后过滤到没有匹配stockExchangeID的行，这就是没有行的原因。是的，它们是连接的，所以如果没有匹配的话，你会从连接的正确部分得到空值，但是设置它似乎是违反直觉的。

假设您想要所有没有特定stock_exchange的公司（包括没有证券交易所的公司），那么首先失去第一个条件并做到：

SELECT COMPANY_NAME
FROM COMPANIES c
LEFT OUTER JOIN STOCK_EXCHANGES_COMPANIES sec 
    ON c.COMPANY_ID = sec.COMPANY_ID
LEFT OUTER JOIN STOCK_EXCHANGES se 
    ON sec.STOCK_EXCHANGE_ID = se.STOCK_EXCHANGE_ID
WHERE se.STOCK_EXCHANGE_ID != [idhere];

要选择有证券交易所而非特定证券交易所的公司，请添加一个条件以检查它是否已加入STOCK_EXCHANGES

SELECT COMPANY_NAME
FROM COMPANIES c
LEFT OUTER JOIN STOCK_EXCHANGES_COMPANIES sec 
    ON c.COMPANY_ID = sec.COMPANY_ID
LEFT OUTER JOIN STOCK_EXCHANGES se 
    ON sec.STOCK_EXCHANGE_ID = se.STOCK_EXCHANGE_ID
WHERE se.STOCK_EXCHANGE_ID != [idhere]
    AND se.STOCK_EXCHANGE_ID IS NOT NULL;

SQL如何使用某些东西！=有效的东西？

2 个答案: