I'm having a bit of trouble debugging a SQL query and would really appreciate some help.
Here is the query:
SELECT p.id, p.type, p.submission_id,
p.title, p.description, p.date, extract('epoch' FROM p.time) AS time,
podcasts.image_url, podcasts.title AS podcast_title,
COUNT(u1) as upvote_count, u2.id as upvote_id,
episodes.mp3_url, episodes.duration,
COUNT(c) as comment_count
FROM posts AS p LEFT JOIN upvotes AS u1 ON p.id=u1.post_id AND u1.comment_id=-1
LEFT JOIN upvotes AS u2 ON p.id=u2.post_id AND u2.user_id=$1 AND u2.comment_id=-1
LEFT JOIN episodes ON p.submission_id = episodes.id
LEFT JOIN podcasts ON episodes.podcast_id=podcasts.id
LEFT JOIN comments AS c ON c.post_id=p.id
WHERE p.type='podcast' AND p.time IS NOT NULL
GROUP BY(p.id, u2.id, podcasts.image_url, episodes.mp3_url, episodes.duration, podcasts.title);
The unexpected behavior comes from the two COUNT statements. I expect upvote_count to be equivalent to
SELECT COUNT(*) FROM upvotes WHERE upvotes.post_id = (individual post id);
for each individual post and same for comment count (which I expect to return the total number of comments for each post. However, I am getting strange seemingly random results from these queries for those two fields. Can anybody help me diagnose the problem?
count() (and all other aggregate functions) ignores null values.
However, COUNT(c) references the complete row ("record") from the table alias c But that is is always not null even when all columns of that record are null.
You need to change both count() calls and pass a column from that table to it, e.g. count(u1.post_id) and count(c.post_id)
Related
I don't quite understand my why code works but how does SQL do the joining process with 3 tables?
Heres the code with the table diagram below:
select category.category_id, name, count(film_id)
from films
right join film_category using (film_id)
right join category using (category_id)
group by category.category_id, name
order by count desc, name
I selected films in the 2nd line and right joined it to film_category, can someone confirm that keeps the 'film_category' information if 'films' doesn't contain the same id? Does SQL just magically know it should join 'film_category' with 'category'? Does that mean I can shuffle the order of joins around then?
Thanks
Most people don't recommend using RIGHT [OUTER] JOIN, for they are usually less readable than LEFT [OUTER] JOIN, as you must read the FROM clause backwards. I, too, advise not to use them.
You select from category, outer join film_category and then outer join films. Thus you select all categories with their films keeping categories in the result that have no associated film.
The query is wrong in only two regards:
Semantical: With USING(film_id) the film_id is never null, so COUNT(film_id) will not count zero when there is no film, but one.
Syntactical: The COUNT in ORDER BY lacks its parameter.
The query would be more readable with left outer joins as mentioned. With ON instead of USING and all columns qualified with their table names and with alias names for readability:
select c.category_id, c.name, count(f.film_id)
from category c
left join film_category fc on fc.category_id = c.category_id
left join films f on f.film_id = fc.film_id
group by c.category_id, c.name
order by count(f.film_id) desc, c.name;
I have three tables:
articles(id,title,message)
comments(id,article_id,commentedUser_id,comment)
comment_likes(id, likedUser_id, comment_id, action_like, action_dislike)
I want to show comments.id, comments.commentedUser_id, comments.comment, ( Select count(action_like) where action_like="like") as likes and comment_id=comments.id where comments.article_id=article.id
Actually I want to count all action_likes that related to any comment. And show all all comments of articles.
action_likes having only two values null or like
SELECT c.id , c.CommentedUser_id , c.comment , (cl.COUNT(action_like) WHERE action_like='like' AND comment_id='c.id') as likes
FROM comment_likes as cl
LEFT JOIN comments as c ON c.id=cl.comment_id
WHERE c.article_id=article.id
It shows nothing, I know I'm doing wrong way, that was just that I want say
I guess you are looking for something like below. This will return Article/Comment wise LIKE count.
SELECT
a.id article_id,
c.id comment_id,
c.CommentedUser_id ,
c.comment ,
COUNT (CASE WHEN action_like='like' THEN 1 ELSE NULL END) as likes
FROM article a
INNER JOIN comments C ON a.id = c.article_id
LEFT JOIN comment_likes as cl ON c.id=cl.comment_id
GROUP BY a.id,c.id , c.CommentedUser_id , c.comment
IF you need results for specific Article, you can add WHERE clause before the GROUP BY section like - WHERE a.id = N
I would recommend a correlated subquery for this:
SELECT a.id as article_id, c.id as comment_id,
c.CommentedUser_id, c.comment,
(SELECT COUNT(*)
FROM comment_likes cl
WHERE cl.comment_id = c.id AND
cl.action_like = 'like'
) as num_likes
FROM article a INNER JOIN
comments c
ON a.id = c.article_id;
This is a case where a correlated subquery often has noticeably better performance than an outer aggregation, particularly with the right index. The index you want is on comment_likes(comment_id, action_like).
Why is the performance better? Most databases will implement the group by by sorting the data. Sorting is an expensive operation that grows super-linearly -- that is, twice as much data takes more than twice as long to sort.
The correlated subquery breaks the problem down into smaller pieces. In fact, no sorting should be necessary -- just scanning the index and counting the matching rows.
I have 3 datasets: company, post, postedited,
I want to count the numbers of companies' post and postedited. some companies post but did not edited.
here is my query :
SELECT company.name, company.id, count(*),
( select count(*)
from post, postedited
where post.id=postedited.post_id)
from company, post as p
where company.id=p.company_id
group by company_id
the outcome of post is right, but the column of postedited is the same. what's wrong with my query?
Your subquery is completely unrelated to the main query. It selects post and postedited and counts. You are showing this result for every row of the main query.
You want the subquery relate to the main query's post. So remove the post table from the subquery's from clause:
(select count(*) from postedited where postedited.post_id = p.id)
Now this subquery selects a count for the post_id of the main query's records. At last you must get the sum of the counts:
select
c.name, c.id, count(*) as posts,
sum(select count(*) from postedited pe where pe.post_id = p.id) as edits
from company c
join post p on p.company_id = c.id
group by c.id;
You can achieve the same thus:
select
c.name, c.id, count(distinct p.id) as posts, count(pe.post_id) as edits
from company c
join post p on p.company_id = c.id
left join postedited pe on pe.post_id = p.id
group by c.id;
SELECT c.name AS companyName
, c.id AS companyID
, COUNT(DISTINCT p.id) AS postCount
, COUNT(DISTINCT pe.post_id) AS postEditCount
FROM company c
LEFT OUTER JOIN post p ON p.Company_ID = c.ID
LEFT OUTER JOIN postEdited pe ON pe.Company_ID = c.ID
GROUP BY c.id, c.name
That will give you a list of all companies in your company table with a count of each of their posts and edited posts. If you need to further query against that dataset, you can. Or you can add a WHERE clause to the above query to filter it.
And I agree, please don't use comma syntax. It's very easy to produce unintended results, and it doesn't give a good representation of what you're actually querying against. Plus, it's no longer standard and being deprecated in many flavors of SQL. Good JOIN syntax will make your life much easier.
I have a Database with the following two tables, member, POSTS I am looking for a way to get the count of how many posts a user has.
(Source: http://i.stack.imgur.com/FDv31.png)
I have tried many variations of the following SQL command with out any success. instead of showing the count of posts for a single user it shows a single row with all the posts as the count.
In the end I want something like this
(Source: http://i.stack.imgur.com/EbaEj.png)
Might be that I'm missing something here, but this query would seem to give you the results you want:
SELECT member.ID,
member.Name,
(SELECT COUNT(*) FROM Posts WHERE member.ID = Posts.user_id) AS total
FROM member;
I have left comment out of the query as it is not obvious what comment you want to be returned in that column for the group of comments that is counted.
See a SQL Fiddle demo here.
Edit
Sorry, misinterpreted your question :-) This query will properly return all the comments, along with the person who posted them and the total number of comments that the person made:
SELECT Posts.ID,
member.Name,
(SELECT COUNT(*) FROM Posts WHERE member.ID = Posts.user_id) AS total,
Posts.comment
FROM Posts
INNER JOIN member ON Posts.user_id = member.ID
GROUP BY Posts.ID, member.Name, member.ID, Posts.comment;
See an updated SQL Fiddle demo here.
You could use a subquery to calculate the total posts per member:
select m.ID
, m.Name
, coalesce(grp.total, 0)
, p.comment
from member m
left join
posts p
on p.user_id = m.id
left join
(
select user_id
, count(*) as total
from posts
group by
user_id
) grp
on grp.user_id = m.id
select
a.id
, a.name
, count(1) over (partition by b.user_id) as TotalCountPerUser
, b.comment
from member a join post b
on a.id = b.user_id
I need to construct some rather simple SQL, I suppose, but as it's a rare event that I work with DBs these days I can't figure out the details.
I have a table 'posts' with the following columns:
id, caption, text
and a table 'comments' with the following columns:
id, name, text, post_id
What would the (single) SQL statement look like which retrieves the captions of all posts which have one or more comments associated with it through the 'post_id' key? The DBMS is MySQL if it has any relevance for the SQL query.
select p.caption, count(c.id)
from posts p join comments c on p.id = c.post_id
group by p.caption
having count (c.id) > 0
SELECT DISTINCT p.caption, p.id
FROM posts p,
comments c
WHERE c.post_ID = p.ID
I think using a join would be a lot faster than using the IN clause or a subquery.
SELECT DISTINCT caption
FROM posts
INNER JOIN comments ON posts.id = comments.post_id
Forget about counts and subqueries.
The inner join will pick up all the comments that have valid posts and exclude all the posts that have 0 comments. The DISTINCT will coalesce the duplicate caption entries for posts that have more then 1 comment.
I find this syntax to be the most readable in this situation:
SELECT * FROM posts P
WHERE EXISTS (SELECT * FROM Comments WHERE post_id = P.id)
It expresses your intent better than most of the others in this thread - "give me all the posts ..." (select * from posts) "... that have any comments" (where exist (select * from comments ... )). It's essentially the same as the joins above, but because you're not actually doing a join, you don't have to worry about getting duplicates of the records in Posts, so you'll just get one record per post.
SELECT caption FROM posts
INNER JOIN comments ON comments.post_id = posts.id
GROUP BY posts.id;
No need for a having clause or count().
edit: Should be a inner join of course (to avoid nulls if a comment is orphaned), thanks to jishi.
Just going off the top of my head here but maybe something like:
SELECT caption FROM posts WHERE id IN (SELECT post_id FROM comments HAVING count(*) > 0)
You're basically looking at performing a subquery --
SELECT p.caption FROM posts p WHERE (SELECT COUNT(*) FROM comments c WHERE c.post_id=p.id) > 1;
This has the effect of running the SELECT COUNT(*) subquery for each row in the posts table. Depending on the size of your tables, you might consider adding an additional column, comment_count, into your posts table to store the number of corresponding comments, such that you can simply do
SELECT p.caption FROM posts p WHERE comment_count > 1