Question

我有两个表格如下所示：

**tblComments**
Commentid
addeddate
Postid


**tblPosts**
Postid
AddedBy
AddedDate
Title

我想找到每篇帖子的总评论。怎么办？

Answer 1

您需要加入表格以查找没有评论的帖子以生成零

select
    count(c.postid), P.postid --edit
from
    tblPosts P 
    LEFT JOIN
    tblComments C On P.postid  = C.postid 
group by
    P.postid

Answer 2

为没有评论的帖子生成零，T-SQL：

select
    P.postid, count(c.postid) -- should not use COUNT(*) for LEFT JOINs
from
    tblPosts P 
    LEFT JOIN
    tblComments C On P.postid  = C.postid 
group by
    P.postid

对于Postgresql，获取行的基数：

select
    P.postid, count(c.*) -- should not use COUNT(*) for LEFT JOINs
from
    tblPosts P 
    LEFT JOIN
    tblComments C On P.postid  = C.postid 
group by
    P.postid

与此相关：http://www.ienablemuch.com/2010/04/debunking-myth-that-countdracula-is.html

这就是不应该在LEFT JOIN上使用COUNT（*）的原因：

create table posts
(
post_id int identity(1,1) not null primary key,
post varchar(max)
);

create table comments
(
post_id int not null references posts(post_id),
comment_id int identity(1,1) not null primary key,
comment varchar(max)
);

insert into posts(post) values('hello');

insert into posts(post) values('oh hai');
insert into comments(post_id,comment) values(SCOPE_IDENTITY(), 1);

-- don't
select p.post_id, COUNT(*) as comment_count from
posts p
left join comments c on c.post_id = p.post_id
group by p.post_id

-- do
select p.post_id, COUNT(c.post_id) as comment_count from
posts p
left join comments c on c.post_id = p.post_id
group by p.post_id

输出：

post_id     comment_count
----------- -------------
1           1
2           1


post_id     comment_count
----------- -------------
1           0
2           1

查询样式在Postgresql上更具吸引力，想到它，我们真正在计算的是集合的基数，而不是列：

-- looks better on Postgresql, COUNT(c.*)
select p.post_id, COUNT(c.*) as comment_count from
posts p
left join comments c on c.post_id = p.post_id
group by p.post_id

Answer 3

select count(*), postid from tblPosts group by postid

应该做我想过的伎俩

Answer 4

编辑：根据您的评论编辑，以获取所有帖子，包括0评论。试试看。我认为这应该像你期望的那样工作

SELECT p.Postid, p.Title, ISNULL(X.count,0)
FROM tblPosts p
LEFT OUTER JOIN 
   (SELECT postid, count(commentid) as Total
   FROM tblComments
   GROUP BY Postid) AS X
ON p.Postid = X.Postid

Answer 5

select case(when count(commentid)>0 then count(commentid) else 0 end), postid from tblPosts group by postid

T-SQL组操作

5 个答案: