mysql - 为什么mysql的where子句中不能使用变量

Question

使用 Mysql 此语句有效:

SELECT *, ABS(UNIX_TIMESTAMP(time) - 
       UNIX_TIMESTAMP('2013-11-14 14:35:21') ) as diff from Alert
order by diff
limit 3

但是如果我添加一个 where 子句如下，它会给出“'where 子句'中的未知列'diff'”错误

SELECT *, ABS(UNIX_TIMESTAMP(time) - 
       UNIX_TIMESTAMP('2013-11-14 14:35:21') ) as diff from Alert
where diff < 2
order by diff
limit 3

好的，我将语句更改为使用having 子句，如下所示:

SELECT *, ABS(UNIX_TIMESTAMP(time) - 
       UNIX_TIMESTAMP('2013-11-14 14:35:21') ) as diff from Alert
order by diff
having diff<2

它仍然给我你的 SQL 语法有错误；检查与您的 MySQL 服务器版本相对应的手册，了解在第 4 行的“having diff<2”附近使用的正确语法。

Answer 1

您不能在 where 子句中使用别名，但使用 order by 就可以了，因为 order by 发生在查询获取完整结果之后。

你的第三个查询给出错误，因为我认为正确的语法是首先有然后排序

SELECT *, ABS(UNIX_TIMESTAMP(time) - UNIX_TIMESTAMP('2013-11-14 14:35:21') ) as diff 
from Alert having diff<2 order by diff 

更多细节

WHERE is used while listing and no ALIAS names are available yet

HAVING filters rows after listing all possible rows so ALIAS names are generated