Question

我有四个表格，结构如下：

panelists（panelist_id，first_name，last_name等。）
projects（project_id，title，created_date等。）
panelists_on_projects（pp_id，panelist_id，project_id等。）
comments（comment_id，panelist_id，project_id等。）

想象一下，我在同一个项目中有四个小组成员（“x”），但只有3个小组成员留下了评论。我试图找出一个查询来计算每个小组成员的评论，并为附加到项目x的小组成员返回零，但是谁没有留下评论。

我尝试使用以下内容：

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

但我只得到实际发表评论的3名小组成员的结果。有什么建议吗？

Answer 1

outer join是关键部分，我也认为您在评论加入中忘记了该项目。按特定列计数时，不会计算空值。

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

这是我的实施。虽然如果你知道project_id，你可以直接进入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

如果没有评论联接的第二个条件and com.project_id = proj.project_id，这将是项目'x'上的小组成员的所有项目的评论总数的计数

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"

查询计算每个小组成员的评论

3 个答案: