I have two tables, for this example, I'm going to call them "content" and "likes".
"Content" contains multiple entries (each one like a famous quote or something of that nature).
The site allows users to click a like button and as such, an entry is inserted into the "likes" table with the corresponding ID from the "content" table so it can traced back.
I wish to write a query that sorts the content of the "content" table by the number of likes it has in the "likes" table - is that possible? If so, how?
Many thanks and happy holidays (the second part applies even if the problem remains unresolved)
Something like:
SELECT c.*
FROM Content c, Likes l
WHERE c.Content_Id = l.Content_Id
GROUP BY c.Content_Id
ORDER BY COUNT(*) DESC
Running a query like that every time you want to show the quotes ordered by the number of likes can become a bit intensive on the database. Have you considered adding a field to the quotes table that only keeps track of the number of likes?
That way, a query like this would suffice:
SELECT quote FROM content ORDER BY likes DESC;
It's somewhat creating redundancy, but not quite. Just an alternative to consider.
Yes it is possible to write a query like that.
SELECT count(*) as total, content.*
FROM content, likes
WHERE content.id = likes.id
GROUP BY total
ORDER BY total desc
Related
Lets start by saying that I cant use INDEXING as I need the INSERT, DELETE and UPDATE for this table to be super fast, which they are.
I have a page that displays a summary of order units collected in a database table. To populate the table an order number is created and then individual units associated with that order are scanned into the table to recored which units are associated with each order.
For the purposes of this example the table has the following columns.
id, UID, order, originator, receiver, datetime
The individual unit quantities can be in the 1000's per order and the entire table is growing to hundreds of thousands of units.
The summary page displays the number of units per order and the first and last unit number for each order. I limit the number of orders to be displayed to the last 30 order numbers.
For example:
Order 10 has 200 units. first UID 1510 last UID 1756
Order 11 has 300 units. first UID 1922 last UID 2831
..........
..........
Currently the response time for the query is about 3 seconds as the code performs the following:
Look up the last 30 orders by by id and sort by order number
While looking at each order number in the array
-- Count the number of database rows that have that order number
-- Select the first UID from all the rows as first
-- Select the last UID from all the rows as last
Display the result
I've determined the majority of the time is taken by the Count of the number of units in each order ~1.8 seconds and then determining the first and last numbers in each order ~1 second.
I am really interested in if there is a way to speed up these queries without INDEXING. Here is the code with the queries.
First request selects the last 30 orders processed selected by id and grouped by order number. This gives the last 30 unique order numbers.
$result = mysqli_query($con, "SELECT order, ANY_VALUE(receiver) AS receiver, ANY_VALUE(originator) AS originator, ANY_VALUE(id) AS id
FROM scandb
GROUP BY order
ORDER BY id
DESC LIMIT 30");
While fetching the last 30 order numbers count the number of units and the first and last UID for each order.
while($row=mysqli_fetch_array($result)){
$count = mysqli_fetch_array(mysqli_query($con, "SELECT order, COUNT(*) as count FROM scandb WHERE order ='".$row['order']."' "));
$firstLast = mysqli_fetch_array(mysqli_query($con, "SELECT (SELECT UID FROM scandb WHERE orderNumber ='".$row['order']."' ORDER BY UID LIMIT 1) as 'first', (SELECT UID FROM barcode WHERE order ='".$row['order']."' ORDER BY UID DESC LIMIT 1) as 'last'"));
echo "<td align= center>".$count['count']."</td>";
echo "<td align= center>".$firstLast['first']."</td>";
echo "<td align= center>".$firstLast['last']."</td>";
}
With 100K lines in the database this whole query is taking about 3 seconds. The majority of the time is in the $count and $firstlast queries. I'd like to know if there is a more efficient way to get this same data in a faster time without Indexing the table. Any special tricks that anyone has would be greatly appreciated.
Design your database with caution
This first tip may seems obvious, but the fact is that most database problems come from badly-designed table structure.
For example, I have seen people storing information such as client info and payment info in the same database column. For both the database system and developers who will have to work on it, this is not a good thing.
When creating a database, always put information on various tables, use clear naming standards and make use of primary keys.
Know what you should optimize
If you want to optimize a specific query, it is extremely useful to be able to get an in-depth look at the result of a query. Using the EXPLAIN statement, you will get lots of useful info on the result produced by a specific query, as shown in the example below:
EXPLAIN SELECT * FROM ref_table,other_table WHERE ref_table.key_column=other_table.column;
Don’t select what you don’t need
A very common way to get the desired data is to use the * symbol, which will get all fields from the desired table:
SELECT * FROM wp_posts;
Instead, you should definitely select only the desired fields as shown in the example below. On a very small site with, let’s say, one visitor per minute, that wouldn’t make a difference. But on a site such as Cats Who Code, it saves a lot of work for the database.
SELECT title, excerpt, author FROM wp_posts;
Avoid queries in loops
When using SQL along with a programming language such as PHP, it can be tempting to use SQL queries inside a loop. But doing so is like hammering your database with queries.
This example illustrates the whole “queries in loops” problem:
foreach ($display_order as $id => $ordinal) {
$sql = "UPDATE categories SET display_order = $ordinal WHERE id = $id";
mysql_query($sql);
}
Here is what you should do instead:
UPDATE categories
SET display_order = CASE id
WHEN 1 THEN 3
WHEN 2 THEN 4
WHEN 3 THEN 5
END
WHERE id IN (1,2,3)
Use join instead of subqueries
As a programmer, subqueries are something that you can be tempted to use and abuse. Subqueries, as show below, can be very useful:
SELECT a.id,
(SELECT MAX(created)
FROM posts
WHERE author_id = a.id)
AS latest_post FROM authors a
Although subqueries are useful, they often can be replaced by a join, which is definitely faster to execute.
SELECT a.id, MAX(p.created) AS latest_post
FROM authors a
INNER JOIN posts p
ON (a.id = p.author_id)
GROUP BY a.id
Source: http://20bits.com/articles/10-tips-for-optimizing-mysql-queries-that-dont-suck/
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
I'm developing a search function for a website. I have a table called keywords with two fields id and keyword. I have two separate search queries for AND and OR. The problem is with the AND query. It is not returning the result that I expect.
The printed SQL is :
SELECT COUNT(DISTINCT tg_id)
FROM tg_keywords
WHERE tg_keyword='keyword_1'
AND tg_keyword='keyword_2'
The count returned is 0, while if I perform the same SQL with OR instead of AND the count returned is 1. I expected the count to be 1 in both cases, and I need it to be this way as the AND results will take priority over the OR results.
Any advice will be much appreciated.
Thanks
Archie
It will always return 0, unless keyword_1=keyword_2. tg_keyword can only have one value, and when you say AND, you're asking for both conditions to be true.
It's the same, logically speaking, as asking "How many friends do I have whose name is 'JACK' and 'JILL'"? None, nobody is called both JACK and JILL.
I don't know what your table looks like and how things are related to each other, but this query makes no sense. You're returning rows where the keyword is one thing and another thing at the same time? That's impossible.
You probably have another table that links to the keywords? You should search with that, using a join, and search for both keywords. We could give you a more precise answer if you could tell us what your tables look like.
EDIT: Based on what you wrote in a comment below (please edit your question!!), you're probably looking for this:
SELECT COUNT(DISTINCT tg_id)
FROM tg_keywords AS kw1, tg_keywords AS kw2
WHERE kw1.tg_id = kw2.tg_id
AND kw1.tg_keyword='keyword_1'
AND kw2.tg_keyword='keyword_2'
your query can't work because you have a condition which is always false so no record will be selected!
tg_keyword='keyword_1' AND tg_keyword='keyword_2'
what are you trying to do? Could you post the columns of this table?
tg_keyword='keyword_1' AND tg_keyword='keyword_2'
Logically this cannot be true, ever. It cannot be both. Did you mean something like:
SELECT * FROM keywords
WHERE tg_keyword LIKE '%keyword_1%' OR tg_keyword LIKE '%keyword_2%'
ORDER BY tg_keyword LIKE '%keyword_1%' + tg_keyword LIKE '%keyword_2%' DESC;
Based on the OP's clarification:
I have a table with multiple keywords with the same id. How can I get more than one keyword compared for the same id, as the search results need to be based on how many keywords from a search array match keywords in the keywords table from each unique id. Any ideas?
I assume you're looking to return search results based on a ranking of how many of the selected keywords are a match with those results? In other words, is the ID field that multiple keywords share the ID of a potential search result?
If so, assuming you pass in an array of keywords of the form {k1, k2, k3, k4}, you might use a query like this:
SELECT ID, COUNT(ID) AS ResultRank FROM tg_keywords WHERE tg_keyword IN (k1, k2, k3, k4) GROUP BY ID ORDER BY ResultRank DESC
This example also assumes a given keyword might appear in the tables multiple times with different IDs (because a keyword might apply to multiple search results). The query will return a list of IDs in descending order based on the number of times they appear with any of the selected keywords. In the given example, the highest rank for a given ID should be 4, meaning ALL keywords apply to the result with that ID...
I think you will need to join tg_keywords to itself. Try playing around with something like
select *
from tg_keywords k1
join tg_keywords k2 on k1.tg_id = k2.tg_id
where k1.tg_keyword = 'keyword_1' and k2.tg_keyword = 'keyword_2'
Try:
SELECT tg_id
FROM tg_keywords
WHERE tg_keyword in ('keyword_1','keyword_2')
GROUP BY tg_id
HAVING COUNT(DISTINCT tg_keyword) = 2
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 :)