I'm having problems with my script which need to select all my posts and related comments.
Right now I've following query:
$sql = "SELECT posts.post_title, posts.post_modified, post_content,update_modified, update_content
FROM posts
LEFT JOIN updates
ON posts.post_ID = updates.update_post_ID";
The query works great besides if the post has multiple comments it gives me multiple entries.
I've searched around but unfortunately I wasn't able to re-script my query for my needs.
I really hope someone can help me out?
I think you want the DISTINCT keyword, used as SELECT DISTINCT ... to avoid duplicates. However if I understand correctly your comments are in the updates table and you're pulling update_modified and update_content into your recordset. So assuming those are (potentially) unique values then DISTINCT will not collapse them down. It might be best to only pull updates.update_post_ID with DISTINCT, then pull whatever you need from updates based on the IDs you retrieve when you need it.
If you want to return only 1 row per post, with all the comments with the post, the easiest way is using GROUP_CONCAT(). This returns a csv of all the column data. Assuming that update_content is the post comments, try something like -
SELECT posts.post_title, posts.post_modified, post_content, GROUP_CONCAT(update_modified), GROUP_CONCAT(update_content)
FROM posts
LEFT JOIN updates
ON posts.post_ID = updates.update_post_ID
GROUP BY updates.update_post_ID
note - GROUP_CONCAT() has a group_concat_max_len default of 1024. If your comments become too long you will want to increase this before running the GROUP_CONCAT() query or the comments will be truncated -
SET [GLOBAL | SESSION] group_concat_max_len = 10240; // must be in multiples of 1024
SELECT id, name
GROUP_CONCAT(comment) AS comment
FROM table
GROUP BY name;
you will also need to be aware of max_allowed_packet as this is the limit you can set var_group_concat_max_len to.
Related
I have a sql statement:
$feed=$conn->prepare("SELECT * FROM posts WHERE post_by=? OR id=? ORDER BY id DESC LIMIT 10");
$feed->bind_param("ii",$friend['id'],$like[id]);
$feed->execute();
The $friend['id'] is the id of a user's friend, $like[id] is the id of a like by the user's friend.
The posts fetched with this query appear in a page.
What I want is I want to know which all posts have been posted by the user's friends (Which all posts have been fetched using $friends['id']) and which all posts have been liked by the user's friends and appear in the feed(Which all posts have been fetched using $like['id'])
I want to know all possibilities I can try to achieve what I want.
I have tried varying my query with UNION ALL but it shows errors and I could'nt achieve what I want.
Currently there are no errors but I want the user to know how this post appeared in the newsfeed.
Hope you all get a good idea about my question and all types of hacks are also accepted as I want in someway to achieve the result I would also agree to change mt query nature.
Please comment for more info.
Thanks in advance.
SELECT *, post_by = ?postId AS post_by_friend
FROM posts
WHERE post_by = ?postId OR
id = ?friendId
ORDER BY id DESC
LIMIT 10
post_by_friend will be 1 if it matched the first condition, otherwise 0. I haven't benchmarked it, but this method should be faster than StuartLC's UNION suggestion.
What you can do is break the query up on its 'OR' clause into a UNION of two separate queries, and add a marker column to indicate whether the row was found by friend or by like:
SELECT *
FROM
(
SELECT *, 'Friend' AS HowFound
FROM posts
WHERE post_by= ?postId
UNION
SELECT *, 'Like' AS HowFound
FROM posts
WHERE id= ?friendId AND post_by <> ?postId
) x
ORDER BY id DESC
LIMIT 10;
You'll want to exclude rows which match both friend and post classifications from one of the selects, otherwise it will be reported twice (or, otherwise your app will need to combine them).
I'm no PHP guru, but I'm sure there is a way to name the parameters to allow the above exclusion.
The derived table is needed to order and restrict the overall result.
I have a voting script which pulls out the number of votes per user.
Everything is working, except I need to now display the number of votes per user in order of number of votes. Please see my database structure:
Entries:
UserID, FirstName, LastName, EmailAddress, TelephoneNumber, Image, Status
Voting:
item, vote, nvotes
The item field contains vt_img and then the UserID, so for example: vt_img4 and both vote & nvotes display the number of votes.
Any ideas how I can relate those together and display the users in order of the most voted at the top?
Thanks
You really need to change the structure of the voting table so that you can do a normal join. I would strongly suggest adding either a pure userID column, or at the very least not making it a concat of two other columns. Based on an ID you could then easily do something like this:
select
a.userID,
a.firstName,
b.votes
from
entries a
join voting b
on a.userID=b.userID
order by
b.votes desc
The other option is to consider (if it is a one to one relationship) simply merging the data into one table which would make it even easier again.
At the moment, this really is an XY problem, you are looking for a way to join two tables that aren't meant to be joined. While there are (horrible, ghastly, terrible) ways of doing it, I think the best solution is to do a little extra work and alter your database (we can certainly help with that so you don't lose any data) and then you will be able to both do what you want right now (easily) and all those other things you will want to do in the future (that you don't know about right now) will be oh so much easier.
Edit: It seems like this is a great opportunity to use a Trigger to insert the new row for you. A MySQL trigger is an action that the database will make when a certain predefined action takes place. In this case, you want to insert a new row into a table when you insert a row into your main table. The beauty is that you can use a reference to the data in the original table to do it:
CREATE TRIGGER Entries_Trigger AFTER insert ON Entries
FOR EACH ROW BEGIN
insert into Voting values(new.UserID,0,0);
END;
This will work in the following manner - When a row is inserted into your Entries table, the database will insert the row (creating the auto_increment ID and the like) then instantly call this trigger, which will then use that newly created UserID to insert into the second table (along with some zeroes for votes and nvotes).
Your database is badly designed. It should be:
Voting:
item, user_id, vote, nvotes
Placing the item id and the user id into the same column as a concatenated string with a delimiter is just asking for trouble. This isn't scalable at all. Look up the basics on Normalization.
You could try this:
SELECT *
FROM Entries e
JOIN Voting v ON (CONCAT('vt_img', e.UserID) = v.item)
ORDER BY nvotes DESC
but please notice that this query might be quite slow due to the fact that the join field for Entries table is built at query time.
You should consider changing your database structure so that Voting contains a UserID field in order to do a direct join.
I'm figuring the Entries table is where votes are cast (you're database schema doesn't make much sense to me, seems like you could work it a little better). If the votes are actually on the Votes table and that's connected to a user, then you should have UserID field in that table too. Either way the example will help.
Lets say you add UserID to the Votes table and this is where a user's votes are stored than this would be your query
SELECT Users.id, Votes.*,
SUM(Votes.nvotes) AS user_votes
FROM Users, Votes
WHERE Users.id = Votes.UserID
GROUP BY Votes.UserID
ORDER BY user_votes
USE ORDER BY in your query --
SELECT column_name(s)
FROM table_name
ORDER BY column_name(s) ASC|DESC
Is it ok to a mysql query inside a while loop using the ID of each row passed to fetch results from another table? OR is there a better way to do it?
$q = $__FROG_CONN__->query("SELECT cms_page.id, cms_page.title, cms_page.slug, cms_page_part.* FROM cms_page LEFT JOIN cms_page_part ON cms_page_part.page_id=cms_page.id WHERE cms_page.parent_id='8'");
$r = $q->fetchAll(PDO::FETCH_ASSOC);
echo '<ul id="project-list">';
foreach ($r as $row) {
echo '<li>';
echo '<img src="<img src="phpThumb/phpThumb.php?src=public/images/'.$row[0].'/th.jpg&w=162" alt="" />';
echo '<div class="p-text">';
echo '<h4>'.$row["location"].'<span>'.$row["project_date"].'</span></h4>';
echo '<p>'.$row["body"].'</p>';
echo '</div>';
echo '</li>';
}
echo '</ul>';
I am trying to pull the project_date, body and location fields from another table where the sql query matches. The title and slug are held in another table. There should only be a maximum of eight or so results but im getting alot more.
The suggestions using IN are fine, but if you are getting the ids from another query, it might be better to combine these two queries into one query using a join.
Instead of:
SELECT id FROM users WHERE age <30
SELECT id, x FROM userinfo WHERE userid IN ($id1, $id2, ..., $idn)
do:
SELECT users.id, userinfo.x
FROM users
LEFT JOIN userinfo ON userinfo.userid = users.id
WHERE age < 30
To reduce the overhead of preforming a query, you may want to look at getting all the data in a single query. In which case you may want to take a look at IN(), e.g.
SELECT * WHERE x IN (1, 2);
There is also BETWEEN()
SELECT * WHERE x BETWEEN 1 AND 2;
See the mysql docs for more information
I would try to build the query in a way where I only need to pass it once. Something like WHERE ID=1 OR ID=2 OR ... Passing multiple queries and returning multiple recordsets is expensive in terms of processing and network traffic.
This will be very inefficient, what you want is to join the tables on the ID
SELECT * FROM table1 LEFT JOIN table2 ON (table1.ID = table2.ID) WHERE condition
Mysql join documentation
This will return one set of rows with all the information you need, returned from both tables.
In a small application / small result set, this might be okay, but it results in a lot of (small) calls to the database.
If you can find an alternative way (perhaps see Yacoby's suggestion?) in which you can do one call to the database, this is probably better.
EDIT
If you are only interested in the IDs from one table, in order to get the correct results out of another table, perhaps a JOIN is what you are looking for?
SELECT t1.fieldA, t1.fieldB
FROM table1 t1
JOIN table2 t2 ON t1.ID = t2.ID
WHERE t2.someField = 'someValue'
Is it ok to a mysql query inside a while loop using the ID of each row passed to fetch results from another table? OR is there a better way to do it?
You should reformulate your query in SQL. Say, put the ids into a memory table and use it in a JOIN:
SELECT *
FROM idtable
JOIN mytable
ON mytable.id = idtable.id
This way, MySQL will make the loops for you but will make them in (usually) more efficient way.
SQL is a language designed to work with sets.
So if you have a set of ids and a table (which is a set of tuples), your first thought should be "how do I apply the set-based operations to these sets using SQL"?
Of course it is possible to run a bunch of simple queries in a loop but this way you just do extra work which SQL engine developers most probably have already done for you (and usually have done it in more efficient way).
You may want to read this article in my blog:
Double-thinking in SQL
I have a programme listing database with all the information needed for one programme packed into one table (I should have split programmes and episodes into their own) Now since there are multiple episodes for any given show I wish to display the main page with just the title names in ascending and chosen letter. Now I know how to do the basic query but this is all i know
SELECT DISTINCT title FROM programme_table WHERE title LIKE '$letter%'
I know that works i use it. But I am using a dynamic image loading that requires a series number to return that image full so how do I get the title to be distinct but also load the series number from that title?
I hope I have been clear.
Thanks for any help
Paul
You can substitute the DISTINCT keyword for a GROUP BY clause.
SELECT
title
, series_number
FROM
programme_table
WHERE title LIKE '$letter%'
GROUP BY
title
, series_number
There are currently two other valid options:
The option suggested by Mohammad is to use a HAVING clause in stead of the WHERE clause this is actually less optimal:
The WHERE clause is used to restrict records, and is also used by the query optimizer to determine which indexes and tables to use. HAVING is a "filter" on the final result set, and is applied after ORDER BY and GROUP BY, so MySQL cannot use it to optimize the query.
So HAVING is a lot less optimal and you should only use it when you cannot use 'WHERE' to get your results.
quosoo points out that the DISTINCT keyword is valid for all listed columns in the query. This is true, but generally people do not recommend it (there is no performance difference *In some specific cases there is a performance difference***)**. The MySQL optimizer however spits out the same query for both so there is no actual performance difference.
Update
Although MySQL does apply the same optimization to both queries, there is actually a difference: when DISTINCT is used in combination with a LIMIT clause, MySQL stops as soon as it finds enough unique rows. so
SELECT DISTINCT
title
, series_number
FROM
programme_table
WHERE
title LIKE '$letter%'
is actually the best option.
select title,series_number from programme_table group by title,series_number having title like '$letter%';
DISTINCT keyword works actually for a list of colums so if you just add the series to your query it should return a set of unique title, series combinations:
SELECT DISTINCT title, series FROM programme_table WHERE title LIKE '$letter%'
Hey thanks for that but i have about 1000 entries with the same series so it would single out the series as well rendering about 999 programmes useless and donot show.
I however found out away to make it unique and show the series number
SELECT * FROM four a INNER JOIN (SELECT title, MIN(series) AS MinPid FROM four WHERE title LIKE '$letter%' GROUP BY title) b ON a.title = b.title AND a.series = b.MinPid
Hopefully it helps anyone in the future and thank you for the replies :)
I have following query:
Select diary_id,
(select count(*)
from `comments` as c
where c.d_id = d.diary_id) as diary_comments
From `diaries` as d
It takes long time (near 0.119415 in my case).
How to make it faster?
I see only one way: Doing additional query for comment number for each row from my main query. But it will be something like doing queries in cycle. Something like:
while ($r = mysql_fetch_array($res))
{
$comments = mysql_query("select count(*) from `comments` where d_id = ".$r['diary_id']);
}
I think this is a bad strategy. Any other advice?
SELECT d.diary_id, count(c.d_id) as diary_comments
FROM diaries d
LEFT OUTER JOIN comments c ON (d.diary_id = c.d_id)
GROUP BY d.diary_id
I seem to have been downvoted because you can actually retreive all the data needed from just the diaries table. I assumed that this was a simplified example and in reality other fields from the diaries table would be required, also this method brings back records which have no comments. If you don't need any of these two things then I would go with the other answer.
It looks like you have all the data you need in the comments table, so I don't see a reason for the join or subquery.
SELECT d_id AS diary_id, COUNT(*) AS diary_comments
FROM `comments`
GROUP BY d_id
Definitely plus one for tomhaigh's solution, a group by is exactly the thing in this situation.
One other option always worth remembering that you've got two choices here. Firstly to calculate the value on every read, and secondly to calculate it on every write and store the result. In the second case you would need an additional field on diaries called 'comments_count', this would need to be incremented on inserting a new comment. Obviously this could be less accurate than working out the count on every read and will slow your writes down. But if read performance is hurting and writes are noticeably less common than reads then this can be a handy way to consider the problem.