T-SQL Group operation

Question

I have two tables as shown below:

**tblComments**
Commentid
addeddate
Postid


**tblPosts**
Postid
AddedBy
AddedDate
Title

I want to find the total comments for each post. How to do?

Answer 1

You need to join the tables to find posts with no comment to generate zero

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

To produce zero for post without comments, 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 

For Postgresql, get the cardinality of rows:

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 

Related to this: http://www.ienablemuch.com/2010/04/debunking-myth-that-countdracula-is.html

This is why COUNT(*) should not be used on LEFT JOINs:

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

Output:

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


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

Query-style is more appealing on Postgresql, come to think of it, what we are really counting is the cardinality of the set, not the column(s):

-- 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 

should do the trick I would have thought

Answer 4

EDIT: Edited based on your comment below to get all posts including those with 0 comments. Try it out. I think this should work as you expect

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