mysql - MySQl - 无法根据“ques_id”列从“ans_desc”列中获取 NULL 值

Question

SELECT ques_id,ans_desc
FROM answer
ORDER BY ans_desc
HAVING ans_desc=0

Answer 1

在子句中使用ans_desc IS NULL谓词WHERE：

SELECT ques_id, ans_desc 
FROM answer 
WHERE ans_desc = 0 OR ans_desc IS NULL
ORDER BY ans_desc ;

这将只为您提供列中包含0或NULL值的那些ans_desc。

独自一WHERE ans_desc = 0人不会给出NULL价值。

Answer 2

当您只需要选择具有 NULL 值的行时，我认为您甚至不需要“按 ans_desc 排序”。

您应该执行以下操作：

SELECT ques_id as `question`, ans_desc as `answer`
FROM answer
WHERE ans_desc is NULL;

此查询的优点是它不使用文件排序。

有关您正在使用的查询，请参阅解释的输出以了解更多信息。

希望有帮助。

Answer 3

0不同于NULL和不同于空

SELECT ques_id,ans_desc
FROM answer
WHERE ISNULL(ans_desc) OR ans_desc = ''
ORDER BY ans_desc