Question

我在这里有这个问题：

SELECT 
    Q.Question_ID, 
    Q.Question, 
    Q.Department, 
    CASE WHEN C.selected = 'true' THEN CAST('true' AS BIT) ELSE CAST('false' AS BIT) END AS Selected 
FROM TPM_Questions_Default AS Q 
LEFT OUTER JOIN customerQuestions AS C 
    ON Q.Question_ID = C.QuestionID 
   AND C.CustomerID = 123456

这将返回TPM_Questions_Default中的所有项目，如果customerQuestions中不存在QuestionID，则bools会自动为假

现在我需要在其他地方重用此查询，但我需要添加另一个需要返回一行的WHERE CLAUSE（AND Q.Question_ID = 2）。

当我将它添加到查询的末尾时，它仍然返回TPM_Questions_Default中的所有项目，我只需要它返回一个。如何调整此查询以执行我想要的操作？

Answer 1

我怀疑你是将它添加到你的JOIN子句中：

SELECT 
    Q.Question_ID, 
    Q.Question, 
    Q.Department, 
    CASE WHEN C.selected = 'true' THEN CAST('true' AS BIT) ELSE CAST('false' AS BIT) END AS Selected 
FROM TPM_Questions_Default AS Q 
LEFT OUTER JOIN customerQuestions AS C 
    ON Q.Question_ID = C.QuestionID 
   AND C.CustomerID = 123456
   AND Q.Question_ID = 2

何时应将其添加为WHERE子句：

SELECT 
    Q.Question_ID, 
    Q.Question, 
    Q.Department, 
    CASE WHEN C.selected = 'true' THEN CAST('true' AS BIT) ELSE CAST('false' AS BIT) END AS Selected 
FROM TPM_Questions_Default AS Q 
LEFT OUTER JOIN customerQuestions AS C 
    ON Q.Question_ID = C.QuestionID 
   AND C.CustomerID = 123456
WHERE Q.Question_ID = 2

SQL LEFT OUTER JOIN查询，需要添加一个WHERE CLAUSE

1 个答案: