SQL查询 - 多个连接

Question

我无法连接两次相同的表，因为返回的结果完全不正确。

下面的查询工作得很好。但是，我想更改它，以便使用Requirement Traced To列中返回的值返回Requirement Type的额外列。

SELECT R.RQ_REQ_ID  as "Requirement Traced From", 
      R.RQ_REQ_NAME as "Requirement Name", 
      RTY.TPR_NAME  as "Requirement Type", 
      RTR.RT_TO_REQ_ID as "Requirement Traced To" 
FROM  REQ R 
LEFT JOIN REQ_TRACE RTR 
ON  R.RQ_REQ_ID = RTR.RT_FROM_REQ_ID, Req_Type RTY 
WHERE  R.RQ_TYPE_ID = RTY.TPR_TYPE_ID 
AND  RTY.TPR_NAME in ('TOM', 'Business Process Map', 'Work Instruction', 'Functional', 'Customer Journey', 'Business') 
ORDER BY 1 

当我第二次用不同的别名添加REQ和REQ_TYPE表我得到数百行返回28我期待的不是。

任何帮助，将不胜感激。

Answer 1

决不使用逗号。 总是使用明确的JOIN语法。

您需要添加额外加入这样的：

SELECT R.RQ_REQ_ID  as "Requirement Traced From", 
     R.RQ_REQ_NAME as "Requirement Name", 
     RTY.TPR_NAME  as "Requirement Type", 
     RTR.RT_TO_REQ_ID as "Requirement Traced To", 
     RTY2.TPR_NAME as "Requirement Type To", 
FROM REQ R LEFT JOIN 
    REQ_TRACE RTR 
    ON R.RQ_REQ_ID = RTR.RT_FROM_REQ_ID LEFT JOIN 
    Req_Type RTY 
    ON R.RQ_TYPE_ID = RTY.TPR_TYPE_ID LEFT JOIN 
    REQ R R2 
    ON R2.RQ_REQ_ID = RTR.RT_TO_REQ_ID LEFT JOIN 
    Req_Type RTY2 
    ON RTY2.TPR_TYPE_ID = R2.RQ_TYPE_ID 
WHERE RTY.TPR_NAME in ('TOM', 'Business Process Map', 'Work Instruction', 'Functional', 'Customer Journey', 'Business') 
ORDER BY 1; 

Answer 2

您需要使用两个join不是编写表名后加入的条件与逗号：在FROM子句中

SELECT R.RQ_REQ_ID  as "Requirement Traced From", 
     R.RQ_REQ_NAME as "Requirement Name", 
     RTY.TPR_NAME  as "Requirement Type", 
     RTR.RT_TO_REQ_ID as "Requirement Traced To" 
FROM  REQ R 
LEFT JOIN REQ_TRACE RTR 
ON  R.RQ_REQ_ID = RTR.RT_FROM_REQ_ID 
LEFT JOIN Req_Type RTY 
ON R.RQ_TYPE_ID = RTY.TPR_TYPE_ID 
    and RTY.TPR_NAME in ('TOM', 'Business Process Map', 'Work Instruction', 'Functional', 'Customer Journey', 'Business') 
ORDER BY 1