Question

使用Mysql这个语句有效：

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

但如果我按如下方式添加一个where子句，它会在'where子句'中给出“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

它还在给我你的SQL语法有错误;检查与MySQL服务器版本对应的手册，以便在第4行的'having diff＆lt; 2'附近使用正确的语法。

Answer 1

你不能在where子句中使用别名，但是它的顺序是有效的，因为在查询获取完整结果之后会发生顺序。

您的第三个查询是错误的，因为我认为正确的语法首先按

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

Answer 2

使用

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

您不能按顺序by（或where）子句使用命名列计算。

为什么变量不能用在mysql的where子句中

2 个答案: