是否存在查询As函数数据的语法？

Question

使用函数后，是否可以在SQL中查询字段名称？

所以例如

select datediff(hh, startdate, enddate) as HoursBetween
from tbl
where HoursBetween > 0

Answer 1

不幸的是，您不能使用与ALIAS子句在同一级别创建的WHERE，因为WHERE子句正在SELECT子句之前执行。你有两个选择，

一，直接在WHERE子句中使用表达式。

select datediff(hh, startdate, enddate) as HoursBetween 
from tbl 
where datediff(hh, startdate, enddate) > 0

二，将语句包装在子查询中。

SELECT *
FROM
(
    select datediff(hh, startdate, enddate) as HoursBetween 
    from tbl 
) ss
WHERE ss.HoursBetween > 0

这是SQL Order of Operation：

• FROM clause
• WHERE子句
• GROUP BY子句
• HAVING条款
• SELECT条款
• ORDER BY子句

Answer 2

SELECT * 
FROM (select datediff(hh, startdate, enddate) as HoursBetween
from tbl) AS I
WHERE HoursBetween > 0

Answer 3

在MySQL上可以使用 HAVING 子句：

SELECT datediff(hh, startdate, enddate) AS HoursBetween FROM tbl
HAVING HoursBetween > 0