Question

我有下表：

SELECT * FROM calls LIMIT 10;
time               |number     |duration|calltype
2011-02-21 19:06:19|11243243569|    NULL|missed
2010-07-12 17:47:14|11234123445|    NULL|missed
2010-07-01 20:55:14|11234123459|      52|placed
2010-07-24 22:43:53|12342345662|     363|received
2010-07-22 01:05:25|11234123412|    NULL|missed
2010-09-16 00:09:03|14321234231|    NULL|missed
2010-10-08 16:31:13|14234234253|    NULL|missed
2010-10-05 17:34:28|15534535234|    NULL|missed
2010-10-22 04:27:15|16127085031|    NULL|missed
2010-10-22 23:46:55|12182308974|    NULL|missed

我希望将NULL的有效期替换为0，但IFNULL似乎不起作用。

SELECT time,number,IFNULL(duration,0),calltype FROM calls LIMIT 10;
time               |     number|IFNULL(duration,0)|calltype
2011-02-21 19:06:19|11243243569|              NULL|missed
2010-07-12 17:47:14|11234123445|              NULL|missed
2010-07-01 20:55:14|11234123459|                52|placed
2010-07-24 22:43:53|12342345662|               363|received
2010-07-22 01:05:25|11234123412|              NULL|missed
2010-09-16 00:09:03|14321234231|              NULL|missed
2010-10-08 16:31:13|14234234253|              NULL|missed
2010-10-05 17:34:28|15534535234|              NULL|missed
2010-10-22 04:27:15|16127085031|              NULL|missed
2010-10-22 23:46:55|12182308974|              NULL|missed

确实，在尝试使用COALESCE() MAX()聚合运算符并发现它们也返回NULL时出现了这个问题。

这里可能会发生什么？如何让聚合忽略NULL并IFNULL工作？

更新

在列上使用typeof命令会产生以下结果：

SELECT duration,typeof(duration) as typ FROM calls LIMIT 10;
duration|typ
    NULL|text
    NULL|text
      52|integer
     363|integer
    NULL|text
    NULL|text
    NULL|text
    NULL|text
    NULL|text
    NULL|text

Answer 1

ifnull函数仅适用于NULL类型的值，而不适用于文本'NULL'。

要将字符串'NULL'映射到其他内容，您可以使用CASE expression：

SELECT time,
       number,
       CASE duration WHEN 'NULL' THEN 0 ELSE duration END AS duration,
       calltype
FROM calls

使用真正的 NULL值会更好。

IFNULL和COALESCE似乎不起作用

1 个答案: