Question

我想从3个日期字段之间的数据库中获取latest_date。现在我使用以下查询，但它不适合我的要求。如何比较3日期字段以获取latest_date？

SELECT DISTINCT e.id, MAX( cc.updated ) AS clinicComment_date, MAX( af.created ) AS attenderInfo_created, e.updated AS event_updated,
(
 CASE 
 WHEN MAX( cc.updated ) > MAX( af.created ) 
 THEN MAX( cc.updated ) 
 WHEN MAX( cc.updated ) < MAX( af.created ) 
 THEN MAX( af.created ) 
 ELSE e.updated
 END
) AS latest_date
FROM Event e
LEFT JOIN EventClinic ec ON e.id = ec.event_id
LEFT JOIN ClinicComment cc ON ec.clinic_id = cc.clinic_id
LEFT JOIN AttendersInfo af ON af.event_id = e.id
WHERE e.status =  'active'
AND (
e.id =43
OR e.id =45
)
GROUP BY e.title
ORDER BY latest_date DESC

由于

Answer 1

我认为您可以使用GREATEST()功能

GREATEST(cc.updated, af.created, e.updated) AS latest_date

这个应该为您提供单行的最新日期

如果您想要所有行之间的最新信息：

MAX(GREATEST(cc.updated, af.created, e.updated)) AS latest_date

与

相同

GREATEST(MAX(cc.updated), MAX(af.created), MAX(e.updated)) AS latest_date

要处理NULL值，您可以使用COALESCE

GREATEST(
    COALESCE(MAX(cc.updated), 0), 
    COALESCE(MAX(af.created), 0),  
    COALESCE(MAX(e.updated), 0) 
) AS latest_date

或者

 MAX(
    GREATEST(
        COALESCE(cc.updated), 0), 
        COALESCE(af.created), 0),  
        COALESCE(e.updated), 0) 
)) AS latest_date

结果应该是相同的，也许它们的性能不同，我不知道

Answer 2

MySQL有一个名为GREATEST()的功能。你可以替换

行

(
 CASE 
 WHEN MAX( cc.updated ) > MAX( af.created ) 
 THEN MAX( cc.updated ) 
 WHEN MAX( cc.updated ) < MAX( af.created ) 
 THEN MAX( af.created ) 
 ELSE e.updated
 END
) AS latest_date

使用GREATEST(MAX(cc.updated), MAX(af.created)) AS latest_date并添加第三个字段。

Answer 3

我可以在Case语句中看到不满足每个条件的问题。 Mysql提供GREATEST函数来返回最大值

试试这个

select DISTINCT e.id,GREATEST(MAX( cc.updated ),MAX( af.created ),e.updated) as Latest_Date
FROM Event e
LEFT JOIN EventClinic ec ON e.id = ec.event_id
LEFT JOIN ClinicComment cc ON ec.clinic_id = cc.clinic_id
LEFT JOIN AttendersInfo af ON af.event_id = e.id
WHERE e.status =  'active'
AND ( e.id =43 OR e.id =45 )
GROUP BY e.title
ORDER BY latest_date DESC

Mysql比较三个日期时间字段

3 个答案: