mysql - sql查询如何使用IF和ELSE语句

Question

我有查询要选择以获得答案值。我需要检查它是回答还是空。我使用 (d.AM_Answer OR null) 它不起作用。它只取空值。所以，我已经像下面这样更改了我的查询，但它不起作用。

SELECT a.QM_ID,
       a.QM_QCM_ID,
       a.QM_Question,
       a.QM_Type,
       a.QM_Parent_Id,
       c.AM_Answer,
       c.AM_Comments
FROM question_master a
INNER JOIN Assessment_master c ON (c.AM_QM_ID = a.QM_ID
                                   AND c.AM_HNM_ID = %d
                                   AND c.AM_HM_ID = %d
                                   AND c.AM_ASM_Local_Id = %@)
WHERE a.QM_Parent_Id = 0
  AND a.QM_Status = 'A'
  AND a.QM_QCM_ID = %@
  AND a.QM_QRM_Id = %@ IF(d.AM_Answer IS NULL) THEN
UNION
SELECT b.QM_ID,
       b.QM_QCM_ID,
       b.QM_Question,
       b.QM_Type,
       b.QM_Parent_Id,
       NULL,
       NULL
FROM question_master b
INNER JOIN Assessment_master d ON ((d.AM_QM_ID = b.QM_Parent_Id
                                    OR d.AM_QM_ID = b.QM_ID)
                                   AND d.AM_HNM_ID = %d
                                   AND d.AM_HM_ID = %d
                                   AND d.AM_ASM_Local_Id = %@)
WHERE b.QM_Parent_Id != 0
  AND b.QM_Status = 'A'
  AND b.QM_QCM_ID = %@
  AND b.QM_QRM_Id = %@ ELSE IF(d.AM_Answer IS NOT NULL) THEN
UNION
SELECT b.QM_ID,
       b.QM_QCM_ID,
       b.QM_Question,
       b.QM_Type,
       b.QM_Parent_Id,
       d.AM_Answer,
       d.AM_Comments
FROM question_master b
INNER JOIN Assessment_master d ON ((d.AM_QM_ID = b.QM_Parent_Id
                                    OR d.AM_QM_ID = b.QM_ID)
                                   AND d.AM_HNM_ID = %d
                                   AND d.AM_HM_ID = %d
                                   AND d.AM_ASM_Local_Id = %@)
WHERE b.QM_Parent_Id != 0
  AND b.QM_Status = 'A'
  AND b.QM_QCM_ID = %@
  AND b.QM_QRM_Id = %@ END IF

Answer 1

您可以将条件 d.answer 不为空来选择非空答案。