SQL LEFT OUTER JOIN查询，需要添加WHERE子句

Question

I have this query here:

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

This returns all items in TPM_Questions_Default and if a QuestionID does not exist in the customerQuestions the bools are false automatically

Now I need to reuse this query else where but I need to add in another WHERE CLAUSE ( AND Q.Question_ID = 2 ) which needs to return one row.

When I add it to the end of the query it still returns all items in TPM_Questions_Default, I just need it to return one. How can I adjust this query to do what I want?

Answer 1

I suspect you are adding it to your JOIN clause:

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

When you should be adding it as a WHERE clause:

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