Mysql

Question

我可以简化这部分查询吗？

CASE WHEN EXISTS(SELECT a_response from answer where a_userID = ? AND a_questionID = q_id)
            THEN (SELECT a_response from answer where a_userID = ? AND a_questionID = q_id) ELSE null END

或完成查询：

SELECT q_id, q_question, q_userID, q_targetID, concat(u_firstname, ' ',u_lastname), 
        CASE WHEN EXISTS(SELECT a_response from answer where a_userID = ? AND a_questionID = q_id)
            THEN (SELECT a_response from answer where a_userID = ? AND a_questionID = q_id) ELSE null END,
        (SELECT count(a_id) FROM answer WHERE a_questionID = q_id),
        (SELECT count(a_id) FROM answer WHERE a_questionID = q_id AND a_response = 1)
    FROM question INNER JOIN user ON q_userID = u_id
    where q_targetID = ?
    ORDER BY q_created DESC LIMIT ?, 10

Answer 1

我不确定您一开始为什么要使用 exists。子查询将正常工作。如果没有行，标量子查询 returns NULL:

SELECT q_id, q_question, q_userID, q_targetID,
       concat(u_firstname, ' ', u_lastname), 
       (SELECT a_response FROM answer WHERE a_userID = ? AND a_questionID = q_id),
       (SELECT count(a_id) FROM answer WHERE a_questionID = q_id),
       (SELECT count(a_id) FROM answer WHERE a_questionID = q_id AND a_response = 1)
FROM question INNER JOIN
     user
     ON q_userID = u_id
WHERE q_targetID = ?
ORDER BY q_created DESC
LIMIT ?, 10;

Mysql - 使用 null 简化查询 EXIST

Mysql - simplify query EXIST with null

exists