I know for a fact this has been asked a few times before, but none of the answered questions relating to this seem to work or are far too confusing for me..
I should probably explain.
I'm trying to create an AJAX script to run to order some results by the number of 'Likes' it has.
My current code is this:
SELECT COUNT(*) AS total, likes.palette_id, palette.*
FROM likes LEFT JOIN palette ON likes.palette_id = palette.palette_id
GROUP BY likes.palette_id
ORDER BY total DESC
Which works fine, however it doesn't list the results with 0 likes for obvious reasons, they don't exist in the table.
I've attached images of the current tables:
Likes table:
http://imgur.com/EGeR3On
Palette table:
http://imgur.com/fKZmSve
There are no results in the likes table until the user clicks 'Like'. It is then that the database gets updated and the palette_id and user_id are inserted.
I'm trying to count how many times *palette_id* occurs in the likes table but also display 0 for all palettes that don't appear in the likes table.
Is this possible? If so, can someone help me out at all?
Thank you
It might not be the exact MySQL syntax (I'm used to SQL Server), but should be pretty straight forward to translate if needed.
SELECT p.*, IFNULL(l.total, 0) AS total
FROM palette p
LEFT JOIN (
SELECT palette_id, COUNT(*) AS total
FROM likes
GROUP BY palette_id
) l
ON l.palette_id = p.palette_id
ORDER BY total
Try this:
SELECT COUNT(likes.palette_id) AS total, palette.palette_id, palette.*
FROM palette LEFT JOIN likes ON likes.palette_id = palette.palette_id
GROUP BY palette.palette_id
ORDER BY total DESC
EDIT:
In regards to the discussion about listing columns that are not in the GROUP BY, there's a good explanation in this MySql documentation page.
MySQL extends the use of GROUP BY so that the select list can refer
to nonaggregated columns not named in the GROUP BY clause. This means
that the preceding query is legal in MySQL. You can use this feature
to get better performance by avoiding unnecessary column sorting and
grouping. However, this is useful primarily when all values in each
nonaggregated column not named in the GROUP BY are the same for each
group. The server is free to choose any value from each group, so
unless they are the same, the values chosen are indeterminate.
In this example, the palette information not added to the GROUP BY will be the same for each group because we are grouping by palette_id so there won't be any issue using palette.*
Your join is written backwards. It should be palette LEFT JOIN likes, because you want all rows in palette and rows in likes, if they exist. The "all rows in palette" will get you a palette_id for the entries there without any matching "likes."
Related
We need to grab the last and newest 20 entries from different tables. However, the GROUP BY statement skips records because we are working with LEFT JOIN on tables.
All these records are linked to unique persons in another table. We store these person's id's in an array for more queries later.
We have a few tables (in which all those person id's are stored) and we want to get them sorted and grouped.
The tables are like this:
SELECT lastRecord+personID FROM t1
SELECT lastRecord+personID FROM t2
SELECT lastRecord+personID FROM t3
SELECT lastRecord+personID FROM t4
WHERE t5.Essential_Column_Name = '1'
GROUP BY personID
ORDER BY 'all the latest entries'
LIMIT 20
With that, the relevance of all the latest entries should be equal.
We do have a timestamp column as well. Perhaps that might work better.
Any input is highly appreciated!
For people looking for an answer on this; this is the right post, answer and update to this Q:
UNION mysql gives weird numbered results
With thanks to all for the ideas and providing the paths to the right solution.
I've got following tables in my MySQL database:
USERS
iduser nick password
ARTICLES
idarticles iduser text
How can I get by one SQL query the list of e.g. 10 top publishers of articles ordering by the count of added articles? Is there any way to do that? I'm using PHP to ask database.
Yes, this should be quite easy via JOIN and COUNT(). Something like the following
SELECT `users`.`iduser`, COUNT(`articles`.`idarticles`) AS `total_articles`
FROM `users`
INNER JOIN `articles` ON `users`.`iduser` = `articles`.`iduser`
GROUP BY `users`.`iduser`
ORDER BY `total_articles` DESC
LIMIT 10
A little explaining:
COUNT() will get you what it says - a count of all relevant entries in articles
INNER JOIN will pair all entries that belong together (defined via ON)
GROUP BY tells SQL that you are interested in various results, each differing by iduser (without this, you'd get all articles counted in the first returned row)
ORDER BY .. DESC is important to get the result in a descending order (so most published first)
LIMIT 10 does exactly that
I have this SQL query here that grabs the 5 latest news posts. I want to make it so it also grabs the total likes and total news comments in the same query. But the query I made seems to be a little slow when working with large amounts of data so I am trying to see if I can find a better solution. Here it is below:
SELECT *,
`id` as `newscode`,
(SELECT COUNT(*) FROM `likes` WHERE `type`="newspost" AND `code`=`newscode`) as `total_likes`,
(SELECT COUNT(*) FROM `news_comments` WHERE `post_id`=`newscode`) as `total_comments`
FROM `news` ORDER BY `id` DESC LIMIT 5
Here is a SQLFiddle as well: http://sqlfiddle.com/#!2/d3ecbf/1
I would recommend adding a total_likes and total_comments fields to the news table which gets incremented/decremented whenever a like and/or comment is added or removed.
Your likes and news_comments tables should be used for historical purposes only.
This strenuous counting should not be performed every time a page is loaded because that is a complete waste of resources.
You could rewrite this using joins, MySQL has known issues with subqueries, especially when dealing with large data sets:
SELECT n.*,
`id` as `newscode`,
COALESCE(l.TotalLikes, 0) AS `total_likes`,
COALESCE(c.TotalComments, 0) AS `total_comments`
FROM `news` n
LEFT JOIN
( SELECT Code, COUNT(*) AS TotalLikes
FROM `likes`
WHERE `type` = "newspost"
GROUP BY Code
) AS l
ON l.`code` = n.`id`
LEFT JOIN
( SELECT post_id, COUNT(*) AS TotalComments
FROM `news_comments`
GROUP BY post_id
) AS c
ON c.`post_id` = n.`id`
ORDER BY n.`id` DESC LIMIT 5;
The reason is that when you use a join as above, MySQL will materialise the results of the subquery when it is first needed, e.g at the start of this query, mySQL will put the results of:
SELECT post_id, COUNT(*) AS TotalComments
FROM `news_comments`
GROUP BY post_id
into an in memory table and hash post_id for faster lookups. Then for each row in news it only has to look up TotalComments from this hashed table, when you use a correlated subquery it will execute the query once for each row in news, which when news is large will result in a large number of executions. If the initial result set is small you may not see a performance benefit and it may be worse.
Examples on SQL Fiddle
Finally, you may want to index the relevant fields in news_comments and likes. For this particular query I think the following indexes will help:
CREATE INDEX IX_Likes_Code_Type ON Likes (Code, Type);
CREATE INDEX IX_newcomments_post_id ON news_comments (post_id);
Although you may need to split the first index into two:
CREATE INDEX IX_Likes_Code ON Likes (Code);
CREATE INDEX IX_Likes_Type ON Likes (Type);
First check for helping indexes on columns id, post_id and type,code.
I assume this is T-SQL, as that is what I am most familiar with.
First I would check indexes. If that looks good, then I'd check statement. Take a look at your query map to see how it's populating your result.
SQL works backward, so it starts with your last AND statement and goes from there. It'll group them all by code, and then type, and finally give you a count.
Right now, you're grabbing everything with certain codes, regardless of date. When you stated that you want the latest, I assume there is a date column somewhere.
In order to speed things up, add another AND to your WHERE and account for the date. Either last 24 hours, last week, whatever.
I have two tables: "users" and "posts." The posts table has a 'post' column and a 'poster_id' column. I'm working on a PHP page that shows the latest posts by everyone, like this:
SELECT * FROM posts WHERE id < '$whatever' LIMIT 10
This way, I can print each result like this:
id: 43, poster_id:'4', post: hello, world
id: 44, poster_id:'4', post: hello, ward
id: 45, poster_id:'5', post: oh hi!
etc...
Instead of the id, I would like to display the NAME of the poster (there's a column for it in the 'users' table)
I've tried the following:
SELECT *
FROM posts
WHERE id < '$whatever'
INNER JOIN users
ON posts.poster_id = users.id LIMIT 10
Is this the correct type of join for this task? Before learning about joins, I would query the users table for each post result. The result should end up looking similar to this:
id: 43, poster_id:'4', name:'foo', post: hello, world
id: 44, poster_id:'4', name:'foo', post: hello, ward
id: 45, poster_id:'5', name:'fee', post: oh hi!
etc...
Thanks for helping in advance.
WHERE clause must come after the FROM clause.
SELECT posts.*, users.* // select your desired columns
FROM posts
INNER JOIN users ON posts.poster_id = users.id
WHERE id < '$whatever'
LIMIT 10
the SQL Order of Operation is as follows:
FROM clause
WHERE clause
GROUP BY clause
HAVING clause
SELECT clause
ORDER BY clause
UPDATE 1
For those column names that exists on both tables, add an ALIAS on them so it can be uniquely identified. example,
SELECT post.colName as PostCol,
users.colName as UserCol, ....
FROM ....
on the example above, both tables has column name colName. In order to get them both, you need to add alias on them so in your front end, use PostCol and UserCol to get their values.
Try:
SELECT *
FROM posts
INNER JOIN users ON posts.poster_id = users.id
WHERE posts.id < '$whatever'
LIMIT 10
Got the syntax a little incorrect.
Should be
SELECT * FROM posts
INNER JOIN users ON posts.poster_id = users.id
WHERE id < '$whatever' LIMIT 10
The answers already given tell you the main reason for your query not working at all (ie the WHERE clause should come after the JOIN clauses), however, I'd like to make a couple of additional points:
I would suggest using an OUTER JOIN for this. It probably won't make much difference, but in the event of a post record having an invalid poster_id, an INNER JOIN will mean the record is dropped from the results, whereas an OUTER JOIN will mean that the record is included, but the values from the users table will be null. I imagine you don't want to ever have an invalid poster_id on the posts table, but broken data does happen even in the best regulated system, and it is helpful in these cases to still get the data from the query.
I would strongly suggest not doing SELECT *, and instead itemising the fields you want to get back from the query. SELECT * has a number of problems, but it's particularly bad when you have multiple tables in the query, because if you have fields with the same name on both tables, (eg id), then it becomes very hard to distinguish which one you're working with, as your PHP recordset won't include the table reference. Itemising the fields may make your query string longer, but it won't make it any slower - if anything it'll be quicker - and it will be easier to work with in the long run.
Neither of these points are essential; the query will work without them (as long as you switch the WHERE clause to after the JOIN), but they may improve your query and hopefully also improve your understanding of SQL.
I'm using an old php script and have an error with this query. Since I'm not experienced with mysql, I couldn't fix it.
"SELECT COUNT(p.postid) AS pid, p.*, t.* FROM ".TABLE_PREFIX."post AS p
INNER JOIN ".TABLE_PREFIX."thread AS t ON (p.threadid = t.threadid) WHERE
p.username='".$uname."'"
the error is
Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP columns is illegal if there is no GROUP BY clause
I hope someone can help me
Like the error says, you can't SELECT an aggregate function, such as COUNT, without grouping rows either explicitly (using GROUP BY) or implicitly (by just selecting the aggregate). To put it in less technical terms - you're telling the database, "Look up all posts by this username, and the threads they belong to, and the number of posts", and the database is answering you, "the number of posts in what?".
So you'll need to be more specific. If what you actually want is:
If you don't actually care about all the individual posts, and you just want the threads and the number of posts by this user per thread, remove the p.* from the SELECT, and add GROUP BY t.threadid to the end of the query.
If you want the total number of posts as well as all the posts and threads, just count the result rows.
If you want the total number of posts by all users in the threads you're selecting, that'll make the query much more complex, because you'll need to join on post again to get the total post count.
If you don't actually want the count at at all, remove it from the query. :)