mysql - 如何使用IF和ELSE语句进行SQL查询

我有查询来选择以获得答案值。我需要检查它是否回答或为空。我使用了（d.AM_Answer OR null），它不起作用。它仅采用空值。因此，我已经更改了查询，如下所示，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

最佳答案

您可以将条件d.answer不为null的位置选择非null的答案。

关于mysql - 如何使用IF和ELSE语句进行SQL查询，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/22059924/