仅在符合条件时才选择行

Question

我是PostgreSQL和数据库查询的新手。

我有一个user_id列表，大学课程采取，日期开始和完成。 某些用户有多个条目，有时缺少开始日期或结束日期（或两者）。

我需要检索用户最长的课程，或者如果缺少开始日期，则需要检索最新的课程。 如果仍有多个选项可用，请在多个选项中随机选择。

例如

• 用户2（下）我想只有“经济与政治”，因为它具有最新的日期;
• 在用户6上，只有“电气和电子工程”，因为它是更长的路线。

我做不工作（我觉得我的脱轨）查询：

(SELECT Q.user_id, min(Q.started_at) as Started_on, max(Q.ended_at) as Completed_on, 
q.field_of_study 
    FROM 
    (select distinct(user_id),started_at, Ended_at, field_of_study 
    from educations 
    ) as Q 
    group by Q.user_id, q.field_of_study) 
    order by q.user_id 

的结果是：

User_id Started_on  Completed_on Field_of_studies 
    2  "2001-01-01" ""    "International Economics" 
    2  ""    "2002-01-01" "Economics and Politics" 
    3  "1992-01-01" "1999-01-01" "Economics, Management of ..." 
    5  "2012-01-01" "2016-01-01" "" 
    6  "2005-01-01" "2009-01-01" "Electrical and Electronics Engineering" 
    6  "2011-01-01" "2012-01-01" "Finance, General" 
    6  ""    ""    "" 
    6  "2010-01-01" "2012-01-01" "Financial Mathematics" 

Answer 1

我觉得这个查询应该做的你需要什么，它依靠计算ended_at和started_at之间的天数差，并且如果started_at为空（使其为非常长的间隔），则使用0001-01-01：

select 
    educations.user_id, 
    max(educations.started_at) started_at, 
    max(educations.ended_at) ended_at, 
    max(educations.field_of_study) field_of_study 
from educations 
join (
    select 
    user_id, 
    max( 
    ended_at::date 
    - 
    coalesce(started_at, '0001-01-01')::date 
) max_length 
    from educations 
    where (started_at is not null or ended_at is not null) 
    group by user_id 
) x on educations.user_id = x.user_id 
    and ended_at::date 
     - 
     coalesce(started_at, '0001-01-01')::date 
     = x.max_length 
group by educations.user_id 
; 

Sample SQL Fiddle