为什么mysql的where子句中不能使用变量

使用Mysql时，此语句起作用：

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

但如果我添加一个where子句，如下所示，它会在“where子句”中给出“未知列'diff'”错误

SELECT*，ABS（UNIX\u时间戳）
UNIX_时间戳（'2013-11-14 14:35:21'）与警报不同
其中diff<2
按差异订购
限制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

SELECT*，ABS（UNIX\u时间戳）
UNIX_时间戳（'2013-11-14 14:35:21'）与警报不同
按差异订购
有不同的用法

您不能在order by（或where）子句中使用命名列计算。
您不能在where子句中使用别名，但在order by中它可以工作，因为order by发生在查询获取完整结果之后

您的第三个查询给出了错误，因为我认为正确的语法是先有，然后按顺序

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

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

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

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