This is an issue that I've deemed impractical to implement but I would like to get some feedback to confirm.
I have a product and users database, where users can like products, the like data is stored in a reference table with just pid and uid.
The client request is to show 3 users who have liked every product in the product listing.
The problem is, its not possible to get this data in one query for the product listing,
How I once implemented and subsequently un-implemented it was to perform a request for the users who have liked the products during the loop through the product list.
ie.
foreach($prods as $row):
$likers = $this->model->get_likers($row->id);
endforeach;
That works, but obviously results in not only super slow product listings, and also creates a big strain on the database/cpu.
The final solution that was implemented was to only show the latest user who has liked it (this can be gotten from a join in the products list query) and have a link showing how many people have liked, and upon clicking on it, opens a ajax list of likers.
So my question is, is there actually a technique to show likers on the product list, or is it simply not possible to execute practically? I notice actually for most social media sites, they do not show all likers on the listings, and do employ the 'click to see likers' method. However, they do show comments per items on the listing, and this is actually involves the same problem doesn't it?
Edit: mock up attached on the desired outcome. there would be 30 products per page.
By reading your comment reply to Alex.Ritna ,yes you can get the x no. of results with per group ,using GROUP_CONCAT() and the SUBSTRING_INDEX() it will show the likers seperated by comma or whatever separator you specified in the query (i have used ||).ORDER BY clause can be used in group_concat function.As there is no schema information is available so i assume you have one product table one user table and a junction table that maintains the relation of user and product.In the substring function i have used x=3
SELECT p.*,
COUNT(*) total_likes,
SUBSTRING_INDEX(
GROUP_CONCAT( CONCAT(u.firstname,' ',u.lastname) ORDER BY some_column DESC SEPARATOR '||'),
'||',3) x_no_of_likers
FROM product p
LEFT JOIN junction_table jt ON(p.id=jt.product_id)
INNER JOIN users u ON(u.id=jt.user_id)
GROUP BY p.id
Fiddle
Now at your application level you just have to loop through the products and split the x_no_of_likers by separator you the likers per product
foreach($prods as $row):
$likers=explode('||',$row['x_no_of_likers']);
$total_likes= $row['total_likes'];
foreach($likers as $user):
....
endforeach;
endforeach;
Note there is a default 1024 character limit set on GROUP_CONCAT() but you can also increase it by following the GROUP_CONCAT() manual
Edit from comments This is another way how to get n results per group, from this you can get all the fields from your user table i have used some variables to get the rank for product group ,used subquery for junction_table to get the rank and in outer select i have filtered records with this rank using HAVING jt.user_rank <=3 so it will give three users records per product ,i have also used subquery for products (SELECT * FROM product LIMIT 30 ) so the first 30 groups will have 3 results for each,for below query limit cannot be used at the end so i have used in the subquery
SELECT p.id,p.title,u.firstname,u.lastname,u.thumbnail,jt.user_rank
FROM
(SELECT * FROM `product` LIMIT 30 ) p
LEFT JOIN
( SELECT j.*,
#current_rank:= CASE WHEN #current_rank = product_id THEN #user_rank:=#user_rank +1 ELSE #user_rank:=1 END user_rank,
#current_rank:=product_id
FROM `junction_table` j ,
(SELECT #user_rank:=0,#current_rank:=0) r
ORDER BY product_id
) jt ON(jt.product_id = p.id)
LEFT JOIN `users` u ON (jt.`user_id` = u.`id`)
HAVING jt.user_rank <=3
ORDER BY p.id
Fiddle n results per group
You should be able to get a list of all users that have liked all products with this sql.
select uid,
count(pid) as liked_products
from product_user
group by uid
having liked_products = (select count(1) from products);
But as data grows this query gets slow. Better then to maintain a table with like counts that is maintained through a trigger or separately. On every like/dislike the counter is updated. This makes it easy to show the number of likes for each product. Then if the actual users that liked that product is wanted do a separate call (on user interaction) that fetches the specific likes for one product). Don't do this for all products on a page until actually requested.
I am assuming the size of both these tables is non-trivially large. You should create a new table (say LastThreeLikes), where the columns would be pid,uid_1,uid_2 and uid_3, indexed by pid. Also, add a column to your product table called numLikes.
For each "like" that you enter into your reference table, create a trigger that also populates this LastThreeLikes table if the numLikes is less than 3. You can choose to randomly update one of the values anyway if you want to show new users once in a while.
While displaying a product, simply fetch the uids from this table and display them back.
Note that you also need to maintain a trigger for the "Unlike" action (if there is any) to re-populate the LastThreeLikes table with a new user id.
Problem
The problem is the volume of data. From the point of view that you need two integer value as a answer you should forget about building a heavy query from your n<->n relations table.
Solution
Generates a storable representation using the file_put_contents() with append option each time a user likes a product. I don't have enough room to write the class in here.
public function export($file);
3D array format
array[product][line][user]
Example:
$likes[1293][1][456]=1;
$likes[82][2][656]=1;
$likes[65][3][456]=1;
.
.
.
Number of users who like this particular product:
$number_users_like_this_product = count($likes[$idProduct]);
All idUser who like this particular product:
$users_like_this_product = count($likes[$idProduct][$n]);
All likes
$all_likes = count($likes);
Deleting a like
This loop will unset the only line where $idProduct and $IdUser you want. Since all the variables are unsigned integer it is very fast.
for($n=1, $n <= count($likes[$idProduct]), $n++)
{
unset($likes[$idProduct][$n][$idUser]);
}
Conclusion
Get all likes will be easy as:
include('likes.php');
P.S If you want to give a try i will be glad to optimize my stuff and share it. I've created the class in 2012.
Related
Let's imagine I have a databases with two tables, Users and Posts. The first table contains a row for each user, the second table a row for each post that users have written. If I want to display a post count on the users' profiles, which of these two strategies work the best:
Every time a user creates a post I UPDATE the Users table, +1 a field PostCount;
When someone visits the profile I simply run a select statement to get a count of post, for example SELECT COUNT(post_id) FROM Posts WHERE id_user = 100;
In the first case I have to UPDATE a table very often, which it could be bad as I believe a table gets locked when doing the update; in the second case I have to run a count every time the user visits a profile. Which poison is the less bitter? Is there any other way?
I would say that it depends on how many times you will display PostCount, especially for a huge amount of Users. If you are going to display it for 1000+ users on a page that will be called a lot of times, then the first solution should be the best. But you need to do transactions to be sure both tables Posts and Users are updated when adding a new post.
Otherwise, the second solution should be enough, but you should use LEFT OUTER JOIN so that you would get both information from Users and Posts table in only one query. Eg:
SELECT *
FROM Users u
LEFT OUTER JOIN (
SELECT user_id , COUNT(*) AS posts_count
FROM Posts
GROUP BY user_id
) p ON p.user_id = u.id
WHERE u.id = :searched_id
(And anyway you should use a Cache system so that you don't have to do the same SQL query for a same page if shown to several users.)
I have a list of subscribers in table Subscribers. Every time they receive a copy of their subscription, a new record is created in Subscriptions_Fulfilments for each Subscribers.ID.
I can create a table showing each Subscriber ID and the number of copies they received with the following query:
SELECT Sub_ID, COUNT(Sub_ID) fcount FROM `Subscriptions_Fulfilments`
GROUP BY Sub_ID
But I need to create a compound query that returns Subscribers along with a column showing the COUNT(Sub_ID) of Subscriptions_Fulfilments.
So I have two questions:
A) How would you make a query to create a table that shows each Subscriber and the number of times they've received their subscription, based on the COUNT of that Subscriber's ID in Subscriptions_Fulfilments?
B) I'm operating under the assumption that a single MySql query accomplishing this would be more efficient than, say, running two queries, the one above and a SELECT * FROM Subscriptions, and combining the resulting arrays in PHP. I have a feeling I know the answer but I'd like to positively learn something new today.
Unfortunately, after too many tries, I'm clearly not good enough at queries for this and I have very little past the above query to show for it. I apologize if this ends up being a dup, I searched long and hard before asking, but it's quite difficult to search precisely for Query help...
Here is a simple example showing the Subscribers ID and the no of subscription they have received. Hope it helps.
Step 1: select the ids from the Subscriber table
Step 2: select the no of counts of subscriptions received by each subscriber.
Step 3: Join both the table ON the basis of ID.
SELECT SubId, noSub FROM
Subscribers sb JOIN (SELECT SubId AS sid, COUNT(*)AS noSub FROM Subscriptions_Fulfilments GROUP BY SubId)AS ss ON sb.SubId = ss.sid
One of the big advantages of a relational database is the ability to do joins and combinations of the data in your tables in a way that allows for this functionality without having to actually store it in a separate table.
You can accomplish this with a subquery like this:
SELECT Subscribers.name, fulfilments.count FROM Subscribers
INNER JOIN (
SELECT id, count(*) as count FROM Subscriptions_Fulfilments
GROUP BY Sub_Id
)fulfilments ON subscribers.id = fulfilments.id
This might not be 100% what you're looking for and I might have messed up your names, but hopefully this will start to get you in the neighborhood of being correct?
Simply try execute this query:
Select distinct Sub_ID, count from (SELECT Sub_ID, COUNT(Sub_ID) fcount FROM Subscriptions_Fulfilments
GROUP BY Sub_ID);
I am wondering if there is easy way to get number of how many rows of distinct values I have (bad explanation, I know)
Example: I have table, which registers views for my blog articles. I want to count, how many have viewed article a and how many b (I have many articles, I want to get top 10 most viewed articles)
So is there an easy way to get this with SQL, at the moment I did it with php arrays, I'm getting all the distinct rows in array, then I get how many of rows there is for every array value, then I sort array and echo first 10, but that is way too many queries, I was wondering, if there is way to do this with 1 query?
select
a.article_id,
a.title,
a.date,
/* Make sure to count a relevant view from the *views* table.
-- This makes sure that count returns 0 instead of 1 when
-- the article isn't viewed yet. */
count(v.article_id) as viewcount
from
Article a
/* Using left join here, to also include articles that are not viewed at all.
-- You can change this to an inner join if you don't want to include those. */
left join ArticleView v on v.article_id = a.article_id
group by
/* Group by article id, so count() actually counts per article. */
a.article_id
order by
/* Place the best viewed articles on top. */
count(v.article_id) desc
/* And return only 10 articles at most. */
limit 10
This query will return 10 articles, even if there are no 10 that have views at all. If you want to only return articles that actually have views, and you don't need other fields from the article table, you can simplify the query a little:
select
v.article_id,
count(v.article_id) as viewcount
from
ArticleView v
group by
v.article_id
order by
count(v.article_id) desc
limit 10
But the advantage of the first query is that you can also add other fields of 'a' to your query result, like the title. So this single query can actually return all the information you need to generate the entire top-10 list, while the second only provides a list of ids.
It is easy to do with sql grouping.
select articleid, count(*) from view_table group by articled
Obviously, you will need to change the tables and fields.
I am building an auction website. Right now, I am building the item description page, that has item details, as well as current bid history. My bids table has a FK of Item_id.
My current query looks something like this:
SELECT bids.Item_id, bids.User_email, bids.Bid_amount, products.*
FROM bids
INNER JOIN products
ON bids.Item_id=products.Item_id;
This returns all of the bid information I need - but also returns the item description for every bid row. I only need the product information once. Is it best to just use two queries on this?
Any help is appreciated
If you need the bids data separately from the products data, then you should use two queries.
One query cannot really be arrange to return different columns for different rows.
SELECT b.Item_id, b.User_email, b.Bid_amount, p.*
FROM bids b
INNER JOIN products p
ON b.Item_id=p.Item_id
WHERE p.Item_id=something;
This will not repeat products..
I have the following statement that finds all the players of a team in the current season. The players are ordered by their handicap. If their handicaps are the same they are ordered by the oldest added_date, meaning newer members are lower down the list.
SELECT players.playerid_p,
players.fname,
players.sname,
players.tel,
players.mob,
players.email,
season_players.captain
FROM season_players
LEFT JOIN players ON (season_players.playerid_f = players.playerid_p)
LEFT JOIN handicaps ON (handicaps.playerid_f = players.playerid_p)
WHERE season_players.seasonid_f = '$currentSeason'
AND season_players.teamid_f = '".$row["teamid_p"]."'
GROUP BY players.playerid_p
ORDER BY handicaps.handicap ASC, handicaps.added_date ASC
The handicaps table can have multiple entries per player for any reviews they have had.
I can't figure out how to make the latest handicap to be used for the ordering (something maybe to do with MAX on added_date?) yet at the same time if two or more handicaps are the same it order them by oldest registered first based on added_date.
In stead of trying to figure out how to satisfy your needs in current query, wouldn't it be an option to create a separate table, let's say "Handicaps_latest", which only stores player_id and required info of latest review. The reason for doing this is because you are only trying to get(columns in your select clause) the information about players and nothing really needed from handicaps table. In this case, a handicaps table with multiple entries per player might not be a good table to join. But considering that those data might be required in other logic, so leave them there and create a branch new table only storing latest review data could be an option for your case. But it requires some extra work apparently, that is, whenever you insert a new entry into your original handicpas table, a particular entry in handicaps_latest needs to be updated.
SELECT x.*
FROM my_table x
JOIN
( SELECT id,MAX(other_column) max_other_column FROM my_table GROUP BY id) y
ON y.id = x.id
AND y.max_other_column = x.other_column;