mysql - 为什么 MySQL 对于明确定义的列显示 Unknown column？

Question

我正在构建一个 SQL 查询以生成一张已接受或延迟关闭的工单表。

SELECT 
    svc_guid,
    svc_priority,
    cast(svc_minsinqueue AS SIGNED)-(pri_svc_accept_hours * 60) AS accept_overdue,
    cast(svc_minselapsed AS SIGNED)-(pri_svc_close_hours * 60) AS closure_overdue
FROM
    svcrequest
LEFT JOIN 
    priorities
ON 
    pri_id=svc_priority
WHERE
    accept_overdue > 0 OR
    closure_overdue > 0
ORDER BY 
    accept_overdue DESC;

结果是:

#1054 - Unknown column 'accept_overdue' in 'where clause' 

与 WHERE 1 相同的精确查询不会产生错误。 (即使使用 ORDER BY accept_overdue DESC)

Answer 1

不能在where子句中使用列名别名
where 子句在 select 子句之前计算，因此数据库引擎无法知道此列别名 要构建正确的查询，您必须复制相关的列代码

SELECT 
    svc_guid,
    svc_priority,
    cast(svc_minsinqueue AS SIGNED)-(pri_svc_accept_hours * 60) AS accept_overdue,
    cast(svc_minselapsed AS SIGNED)-(pri_svc_close_hours * 60) AS closure_overdue
FROM
    svcrequest
LEFT JOIN 
    priorities
ON 
    pri_id=svc_priority
WHERE
    cast(svc_minsinqueue AS SIGNED)-(pri_svc_accept_hours * 60)  > 0 OR
    cast(svc_minselapsed AS SIGNED)-(pri_svc_close_hours * 60) > 0
ORDER BY 
accept_overdue DESC;

如您所见，ORDER BY 子句将事实评估为最后一个 .. 可以使用列别名