I have two tables: posts and comments. The identifier column in posts is post_id. Each comment has also post_id to point to the responded post.
posts comments
- post_id - comment_id
- message - message
- time - time
- post_id
What I need: Select posts, and also the comments posted on each selected post.
What I do now: I have two queries.
First query to select posts, then I loop thru it, save all post_ids into an array.
Then second query to select all comments based on the saved post_ids from previous step.
Then I loop the comments and adds them to the posts variable.
What I want to do and don't know how: get posts and comments in one query.
Try with -
select * from posts join comments on posts.post_id = comments.post_id
Hope will do the trick.
You can use join concept. For more information see http://www.sitepoint.com/understanding-sql-joins-mysql-database.
Use join clause:
SELECT *
FROM posts AS p,
comments AS c
INNER JOIN posts ON p.post_id = c.post_id;
Related
Let's say I have posts and categorizations.
Post(id)
Categorization(post_id, topic_id)
I'd like to fetch posts that don't belong to a specific topic id.
In my case I have to use an inner join when joining Post to Categorizations as i have other filters to execute.
How do I go about this?
I have tried the following:
Post.joins(:categorizations).where("categorizations.topic_id != ?", doomed_topic_id)
But this returns posts that still have OTHER topics. it only works with posts with just one single topic that happens to be the unwanted one.
For instance, if I have a post with 2 categories (the doomed topic_id AND another topic) this query fails and actually fetches it, instead of filtering it out.
Try:
Posts.where('not exists
(select * from categorizations
where post_id = posts.id and
topic_id = ?)', doomed_id)
You know how the Facebook home feed lists all the recent posts? It shows the user that posted, their actual post and then the first few comments attached to that post. That's what I'm trying to achieve, but I'm having a hard time building a single query that can gather all that data.
I have 3 tables: Uses, Posts and Comments. Each has a unique ID, but they reference each other's IDs. i.e, the Comments table has columns for the user_id of the user who posted and the post_id of the post it is attached to.
At the minute I'm querying SQL to gather all the posts. I join my Users and Comments tables to learn the Username of the poster and a total of how many comments the post has, like so:
$query = "
SELECT `posts`.`id`,`posts`.`message`,`posts`.`link`,
`posts`.`posted`,`posts`.`category`,`posts`.`user_id`,
`users`.`username`,
count(`comments`.`id`)
FROM `posts`
INNER JOIN `users`
ON `posts`.`user_id`=`users`.`id`
JOIN `comments`
ON `comments`.`post_id`=`posts`.`id`
WHERE `posts`.`group_id` = '$id'
AND `posts`.`category`='$filter'
GROUP BY `posts`.`id`
ORDER BY `posts`.`posted`
DESC
";
But instead of finding how many comments a post has, I would instead like to read the first few posts. Can anyone think of a way to achieve this with just the one query?
You can use the LIMIT clause to the the "first" posts. By making the "first posts" a subquery and then joining in the comments you can get everything in a single query. The comments should be left-joined in case of posts with no comments.
Notes:
This query is untested, but it should be close.
This will get all comments for the first few posts, so you'll need to limit the display of "max 3 comments per post" using the front-end display code.
It may be possible to limit comments to 3 per post within this query using variables, but that's not something I know how to do.
Here's the query:
SELECT
FirstPosts.id,
FirstPosts.message,
FirstPosts.link,
FirstPosts.posted,
FirstPosts.posts,
FirstPosts.user_id,
FirstPosts.username,
comments.<< your comment column >>
FROM (
SELECT `posts`.`id`,`posts`.`message`,`posts`.`link`,
`posts`.`posted`,`posts`.`category`,`posts`.`user_id`,
`users`.`username`
FROM `posts`
INNER JOIN `users`
ON `posts`.`user_id`=`users`.`id`
WHERE `posts`.`group_id` = '$id'
AND `posts`.`category`='$filter'
ORDER BY `posts`.`posted` DESC
LIMIT 20) FirstPosts
LEFT JOIN comments ON FirstPosts.id = comments.post_id
ORDER BY FirstPosts.Posted, comments.<< column you use to determing comment order >>
If you determine comment order by a date or sequence, you'll have to ORDER BY FirstPosts.Posted, comments.whatever DESC.
Hope this helps!
I'm having some problems with a query I'm writing. This seems like table structure that is very frequent so I'd love some help.
Let's say I have 3 tables similar to a facebook structure. Users, Wall Posts, and Comments. Users can make wall posts, and comment on other wall posts.
On a users page I would like to show a users wall posts and a count of how many comments that post has. This is what I have so far
I query the Wall Post table using the users id as an inner join to the User table. That gives me a result set of wall posts for that user's page. Then I loop through that result set, take the Wall Post id from each result set, and query the Comment table for the Count of comments for that Wall Post Id. This works, however I have to hit the db twice. Can anyone think of a way that I could do this with one query?
First Query Example:
SELECT wallPost.*, user.currentDefault, user.displayName, user.userName
FROM wallPost
INNER JOIN user ON user.id = wallPost.sourceUserId
WHERE wallPost.recipientId = ? ORDER BY wallPost.id DESC
Second Query Example:
SELECT COUNT(id) AS count
FROM comment
WHERE wallPostId = ?
I would add the count as a subquery and join the subquery to the main query
SELECT
wallPost.*,
user.currentDefault,
user.displayName,
user.userName,
wallpost_commentcount.total
FROM
wallPost
INNER JOIN user ON user.id=wallPost.sourceUserId
LEFT JOIN (SELECT wallPostId,COUNT(*) as total FROM comment GROUP BY wallPostId) as wallpost_commentcount ON (wallpost_commentcount.wallPostId=wallPost.id)
WHERE
wallPost.recipientId = ?
ORDER BY wallPost.id DESC
Please make sure you have an index on comment.wallPostId otherwise this query will take a long time.
I used the LEFT JOIN because you always want to get the wallPost even if there are no comments records yet
I have two tables Posts and comments
Post Table
Post_id
Post_content
Comments table
comment_id
Comment
post_id
created_date
One post can have multiple comments or zero comments
My requirement is getting the latest comment for the posts using left outer join .
I mean result should be one record for the post with the below columns .
post_id,post_content ,comment_id,comment
In simple words posts should be getting along with their latest comment if it exists.
( Currently the system is getting the posts first and then going to the server again to get the latest comments to display , thought of getting them in one shot since we are displaying only one comment initially ... Not sure what should be the best approach if wants to display more than one comment ..?)
Thank You
Regards
Kiran
SELECT Post.post_id, post_content, comment_id, comment
FROM
Post LEFT JOIN Comments
ON Post.post_id = Comments.post_id
AND created_date = (
SELECT MAX(created_date)
FROM Comments
WHERE Post.post_id = Comments.post_id
)
BTW, you should consider indexing Comments {post_id, created_date} for optimal performance, but be mindful of the non-primary key index overhead in case you are using InnoDB (see the "Disadvantages of clustering" section in this article).
I am trying to build a sql query that I think it involves inner joins, but I can't figure it out. Here's the model:
There's two tables: comments, posts
Among many columns, there's the following important ones: comments.id, comments.user_id (owner), comments.post_id (reference to posts table), posts.id, posts.editor_id (which is the person, ie, owner of post).
I want to get the comments that either
1) current user has written, so something like:
select * from comments where user_id = <<current_user_id>>
2) (Assume current_user is editor). Get all comments that belong to a post that you have created.
This is what I have, but I get multiple lines....what am I missing?
select * FROM comments INNER JOIN posts ON comments.post_id = <<test_id>>
WHERE posts.editor_id = <<current_user.editor_id>>;
If you could give me a sql query that includes both of these things, that would be amazing.
Thanks!
Im not sure what you mean by "multiple lines" as you would return a row for each comment.
This should do it.
SELECT comments.*
FROM comments
INNER JOIN posts
ON comments.post_id = posts.id
WHERE posts.editor_id = #editorID;
I want to get the comments that either 1) current user has written, so something like: select * from comments where user_id = <>
That's correct. What's wrong with that?
Get all comments that belong to a post that you have created. This is what I have, but I get multiple lines
Well... you're supposed to. Looking at the schema it's indeed a post could contain many comments. The query given by tyrongower should do what you want.
Going with your requirements, this should work for you:
SELECT * FROM comments
WHERE (user_id = 1)
OR (post_id IN (SELECT id FROM post WHERE editor_id = 1))
In the above query "1" is used as a sample and should be replace with the id of current user.