mysql为导出的max函数列添加条件

Question

我有以下 sql，它不适用于 where 条件。

SELECT t.PROJID,sum(t.UNITS) AS totalunits, sum(t.COST) AS totalcost, project, max(t.DATE) as lastupdated, PROJCODE 
FROM `projectcosts` `t` 
left join projects on projects.PROJID = t.PROJID 
where lastupdated LIKE '%2014-06-11%' 
GROUP BY t.PROJID

Answer 1

您不应在日期上使用 like。正确的解决方案是简单地截断日期并进行比较。但是，您需要使用数据中的字段:

SELECT t.PROJID, sum(t.UNITS) AS totalunits,
       sum(t.COST) AS totalcost, project, max(t.DATE) as lastupdated, PROJCODE 
FROM `projectcosts` `t` left join
     projects on projects.PROJID = t.PROJID 
where date(t.date) = date('2014-06-11')
GROUP BY t.PROJID;

或者，如果您在 lastupdated 上有索引，则使用范围允许使用索引:

SELECT t.PROJID, sum(t.UNITS) AS totalunits,
       sum(t.COST) AS totalcost, project, max(t.DATE) as lastupdated, PROJCODE 
FROM `projectcosts` `t` left join
     projects on projects.PROJID = t.PROJID 
where t.date >= date('2014-06-11') and t.date < date('2014-06-12')
GROUP BY t.PROJID;

您可能确实想要使用having，这是通过使用列别名而不是基本列来建议的。在这种情况下:

SELECT t.PROJID, sum(t.UNITS) AS totalunits,
       sum(t.COST) AS totalcost, project, max(t.DATE) as lastupdated, PROJCODE 
FROM `projectcosts` `t` left join
     projects on projects.PROJID = t.PROJID 
GROUP BY t.PROJID
HAVING date(lastupdated) = date('2014-06-11');