如何使用IF和ELSE语句进行sql查询

Question

我有查询可供选择，以获取答案。我需要检查它是否应答或为空。我使用了(d.AM_Answer或null)它不工作。它只接受空值。所以，我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

1)要检查空值，可以使用COALESCE运算符。

2)也可以使用SQL CASE语句

3)我建议您在准备动态查询时使用存储过程

Answer 2

您可以将where条件d.answer is not null放在其中以选择非null答案。

Answer 3

试一试

IF(d.AM_Answer IS null)

而不是

IF(d.AM_Answer == null)