Oracle SQL查询根据值是否存在返回1或0

Question

I have 2 table Question and Answer tables. Question table has

ID Question_Text
1  Question1
2  Question2
3  Question3

Answer table

Question_ID Answer_text Answered_by
1           Ans1        User1
1           Ans2        User2
1           Ans3        User3
2           Ans4        User1

I need to find out which all questions were answered already? Desired outcome

ID Question_Text Answered
1  Question1     true/1
2  Question2     true/1
3  Question3     false/0

I have written the below query to find whether the question is answered by using EXISTS. I need answered value true/false and also want to fine tune this query so that it checks for first occurrence in answer table and get the value as true/false, instead of running through the thousands of value in answer table.

SELECT * 
FROM QUESTIONS A 
WHERE  EXISTS (SELECT * 
               FROM ANSWERS B 
               WHERE A.QUESTION_ID = B.QUESTION_ID);

Answer 1

A simple LEFT JOIN will do:

SELECT  Q.*, 
        CASE WHEN A.Question_ID IS NULL THEN 0 ELSE 1 END Answered
FROM Question Q
LEFT JOIN ( SELECT DISTINCT Question_ID
            FROM Answer) A
    ON Q.ID = A.Question_ID;

Answer 2

Try: SELECT Q.*, CASE WHEN EXISTS ( SELECT * FROM ANSWERS A WHERE A."Question_ID" = Q.ID ) THEN 'true/1' ELSE 'false/0' END Answered FROM QUESTIONS Q ORDER BY ID
This has the advantage of not having to DISTINCT ANSWERS first. If ANSWERS is big and has an index on Question_ID it may be faster, especially for selected questions. It's still not a proper semi-join solution. The query optimiser could surprise with both this and the @Lamak solution using real data.