MySQl - 无法根据“ques_id”列从“ans_desc”列获取NULL值

Question

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

Answer 1

使用ans_desc IS NULL子句中的WHERE谓词：

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

这只会为您提供0列中NULL或ans_desc值的广告。

仅WHERE ans_desc = 0不会提供NULL值。

Answer 2

0与NULL不同，与空

不同

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

Answer 3

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filesort。

请参阅您正在使用的查询的解释输出。

希望有所帮助。