僅在符合條件時才選擇行

Question

我是PostgreSQL和數據庫查詢的新手。

我有一個user_id列表，大學課程採取，日期開始和完成。 某些用戶有多個條目，有時缺少開始日期或結束日期（或兩者）。

我需要檢索用戶最長的課程，或者如果缺少開始日期，則需要檢索最新的課程。 如果仍有多個選項可用，請在多個選項中隨機選擇。

例如

• 用戶2（下）我想只有「經濟與政治」，因爲它具有最新的日期;
• 在用戶6上，只有「電氣和電子工程」，因爲它是更長的路線。

我做不工作（我覺得我的脫軌）查詢：

(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 

的結果是：

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

我覺得這個查詢應該做的你需要什麼，它依靠計算ended_at和started_at之間的天數差，並且如果started_at爲空（使其爲非常長的間隔），則使用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