查詢計數

Question

我有四個表結構如下每個小組成員的意見：

1. panelists（panelist_id，first_name，last_name等）
2. projects（project_id，title，created_date等。 ）
3. panelists_on_projects（pp_id，panelist_id，project_id等）
4. comments（comment_id，panelist_id，project_id等）

想象一下我對同一個項目（ 「X」）4名專業評委，但只有小組成員3都留下了評論。我試圖找出一個查詢來計算每個小組成員的評論，併爲附加到項目x的小組成員返回一個零，但誰沒有留下評論。

我嘗試使用以下內容：

SELECT first_name, last_name, COUNT(comment_id) 
from panelists 
INNER JOIN comments USING (panelist_id) 
WHERE project = x 

但我只得到了3名誰實際上已經給出了評語結果。有什麼建議麼？

Answer 1

outer join是關鍵部分，我也覺得你在評論連接中忘記了項目。當您通過特定列進行計數時，不會計算空值。

select 
    pj.project_id, 
    p.panelist_id, 
    p.firstname, 
    p.lastname, 
    totalcomments = count(c.comment_id) 
from 
    project pj 
    inner join panelists_on_projects pop on pop.project_id = pj.project_id 
    inner join panelist p on p.panelist_id = pop.panelist_id 
    left outer join comments c on 
     c.panelist_id = p.panelist_id 
     and c.project_id = pj.project_id 
where 
    pj.title = 'X' 
group by 
    pj.project_id, 
    p.panelist_id, 
    p.firstname, 
    p.lastname 

Answer 2

這是我的實現。儘管如果你知道project_id，你可以直接進入panelists_on_projects表。

select 
    pan.first_name 
    , pan.last_name 
    , count(com.comment_id) 
from 
    projects proj 
    inner join panelists_on_project pop 
     on proj.panelist_id = pop.panelist_id 
    inner join panelist pan 
     on pop.panelist_id = pan.panelist_id 
    left outer join comments com 
     on pan.panelist_id = com.panelist_id 
     and com.project_id = proj.project_id 
where 
    proj.title = 'x' 
group by 
    pan.first_name 
    , pan.last_name 

不上的評論二號條件and com.project_id = proj.project_id參加這個會的意見對那名項目「X」

Answer 3

SELECT P.first_name, P.last_name, COUNT(C.comment_id) 

FROM panelists as P 

LEFT JOIN panelists_on_projects AS PP ON PP.panelist_id = P.panelist_id 

LEFT JOIN projects AS PR PN PP.project_id = PR.project_id 

LEFT OUTER JOIN comments As C ON C.panelist_id = P.panelist_id AND C.project_id = PR.project_id 

WHERE PR.title = "x"