Mysql 如何使用IF和ELSE语句进行sql查询

我要选择查询以获取答案值。我需要检查它是否回答或为空。我用了d.AM_-Answer或null，它不起作用。它只接受空值。因此，我改变了我的查询，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

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1要检查空值，可以使用运算符

2您也可以使用SQL CASE语句

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3我建议您在准备动态查询时为此使用存储过程

您可以将条件d.answer设置为非空以选择非空答案。

尝试

IF(d.AM_Answer IS null)

而不是

IF(d.AM_Answer == null)

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from：您不能使用算术比较运算符，例如=，@mcalex，他可以使用的是null。我想它会告诉您一个错误，因为union关键字在这两种情况下，selects之间有什么区别？您能告诉我查询中每个union的用途吗？