sql - 是否有语法来查询作为数据功能的字段？

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ALIAS与子句在同一级别上创建的那个，WHERE因为WHERE子句在子句之前首先执行SELECT。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 子句
• WHERE 子句
• GROUP BY 子句
• HAVING 子句
• 选择子句
• ORDER BY 子句

Answer 2

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

Answer 3

在这种情况下可以使用On MySQL HAVING子句：

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