I want to request 5 random rows from my SQL table using php.
for instance, i need to:
mysql_query("SELECT * FROM catalogue >> not sure what goes here << LIMIT 5");
SELECT * FROM catalogue order by RAND() LIMIT 5
Edit:
For what its worth, Please note that using rand() on a table with large number of rows is going to be slow. This can actually crash your server.
Some Solution:
MediaWiki uses an interesting trick (for Wikipedia's Special:Random feature): the table with the articles has an extra column with a random number (generated when the article is created). To get a random article, generate a random number and get the article with the next larger or smaller (don't recall which) value in the random number column. With an index, this can be very fast. (And MediaWiki is written in PHP and developed for MySQL.)
But this is for a single random row only.
Assuming the table has AUTO INCREMENT on you could get the biggest ID with
SELECT id FROM catalogue ORDER BY id DESC LIMIT 1
and the smallest ID with
SELECT id FROM catalogue ORDER BY id ASC LIMIT 1
making it possible to do this
$randomId = mt_rand($smallestId, $biggestId);
$randomSql = mysql_query("SELECT * FROM catalogue WHERE id='$randomId'");
For five rows you could create the random ID five times.
If you're selecting random rows from a very large table, you may want to experiment with the approaches in the following link:
http://akinas.com/pages/en/blog/mysql_random_row/
note: just to share other options with everyone
Related
i was using order by rand() to generate random rows from database without any issue but i reaalised that as the database size increase this rand() causes heavy load on server so i was looking for an alternative and i tried by generating one random number using php rand() function and put that as id in mysql query and it was very very fast since mysql was knowing the row id
but the issue is in my table all numbers are not availbale.for example 1,2,5,9,12 like that.
if php rand() generate number 3,4 etc the query will be blank as there is no id with number 3 , 4 etc.
what is the best way to generate random numbers preferable from php but it should generate the available no in that table so it must check that table.please advise.
$id23=rand(1,100000000);
SELECT items FROM tablea where status='0' and id='$id23' LIMIT 1
the above query is fast but generate sometimes no which is not availabel in database.
SELECT items FROM tablea where status=0 order by rand() LIMIT 1
the above query is too slow and causes heavy load on server
First of, all generate a random value from 1 to MAX(id), not 100000000.
Then there are at least a couple of good solutions:
Use > not =
SELECT items FROM tablea where status='0' and id>'$id23' LIMIT 1
Create an index on (status,id,items) to make this an index-only query.
Use =, but just try again with a different random value if you don't find a hit. Sometimes it will take several tries, but often it will take only one try. The = should be faster since it can use the primary key. And if it's faster and gets it in one try 90% of the time, that could make up for the other 10% of the time when it takes more than one try. Depends on how many gaps you have in your id values.
Use your DB to find the max value from the table, generate a random number less than or equal to that value, grab the first row in which the id is greater than or equal to your random number. No PHP necessary.
SELECT items
FROM tablea
WHERE status = '0' and
id >= FLOOR(1 + RAND() * (SELECT MAX(id) FROM tablea))
LIMIT 1
You are correct, ORDER BY RAND() is not good solution if you are dealing with large datasets. Depending how often it needs to be randomized, what you can do is generate a column with a random number and then update that number at some predefined interval.
You would take that column and use it as your sort index. This works well for a heavy read environment and produces predicable random order for a certain period of time.
A possible solution is to use limit:
$id23=rand(1,$numberOfRows);
SELECT items FROM tablea where status='0' LIMIT $id23 1
This wont produce any missed rows (but as hek2mgl mentioned) requires knowing the number of rows in the select.
Lets say I have a table T.
And T contains X tuples. (And table T has a numeric PRIMARY KEY)
Is there a way in SQL to get a random subset(n) of these tuples.
That is every time I run the query a different group of tuples is output.
(Note: I know it can be done via the programming language but that would mean running N queries to my database).
The only solution I could come up with was
1.generate n unique random numbers in an array(arr)
2.Add the numbers in a loop as
"SELECT * FROM T where id="+arr[0] + "OR id=" +arr[1].....+"OR id="+arr[n]
AND I'M USING A PHPMYADMIN database
But this seems unelegent to me.Thoughts?
Thanks
SELECT * FROM T ORDER BY RAND() LIMIT 5
I'm trying to get 4 random results from a table that holds approx 7 million records. Additionally, I also want to get 4 random records from the same table that are filtered by category.
Now, as you would imagine doing random sorting on a table this large causes the queries to take a few seconds, which is not ideal.
One other method I thought of for the non-filtered result set would be to just get PHP to select some random numbers between 1 - 7,000,000 or so and then do an IN(...) with the query to only grab those rows - and yes, I know that this method has a caveat in that you may get less than 4 if a record with that id no longer exists.
However, the above method obviously will not work with the category filtering as PHP doesn't know which record numbers belong to which category and hence cannot select the record numbers to select from.
Are there any better ways I can do this? Only way I can think of would be to store the record id's for each category in another table and then select random results from that and then select only those record ID's from the main table in a secondary query; but I'm sure there is a better way!?
You could of course use the RAND() function on a query using a LIMIT and WHERE (for the category). That however as you pointed out, entails a scan of the database which takes time, especially in your case due to the volume of data.
Your other alternative, again as you pointed out, to store id/category_id in another table might prove a bit faster but again there has to be a LIMIT and WHERE on that table which will also contain the same amount of records as the master table.
A different approach (if applicable) would be to have a table per category and store in that the IDs. If your categories are fixed or do not change that often, then you should be able to use that approach. In that case you will effectively remove the WHERE from the clause and getting a RAND() with a LIMIT on each category table would be faster since each category table will contain a subset of records from your main table.
Some other alternatives would be to use a key/value pair database just for that operation. MongoDb or Google AppEngine can help with that and are really fast.
You could also go towards the approach of a Master/Slave in your MySQL. The slave replicates content in real time but when you need to perform the expensive query you query the slave instead of the master, thus passing the load to a different machine.
Finally you could go with Sphinx which is a lot easier to install and maintain. You can then treat each of those category queries as a document search and let Sphinx randomize the results. This way you offset this expensive operation to a different layer and let MySQL continue with other operations.
Just some issues to consider.
Working off your random number approach
Get the max id in the database.
Create a temp table to store your matches.
Loop n times doing the following
Generate a random number between 1 and maxId
Get the first record with a record Id greater than the random number and insert it into your temp table
Your temp table now contains your random results.
Or you could dynamically generate sql with a union to do the query in one step.
SELECT * FROM myTable WHERE ID > RAND() AND Category = zzz LIMIT 1
UNION
SELECT * FROM myTable WHERE ID > RAND() AND Category = zzz LIMIT 1
UNION
SELECT * FROM myTable WHERE ID > RAND() AND Category = zzz LIMIT 1
UNION
SELECT * FROM myTable WHERE ID > RAND() AND Category = zzz LIMIT 1
Note: my sql may not be valid, as I'm not a mySql guy, but the theory should be sound
First you need to get number of rows ... something like this
select count(1) from tbl where category = ?
then select a random number
$offset = rand(1,$rowsNum);
and select a row with offset
select * FROM tbl LIMIT $offset, 1
in this way you avoid missing ids. The only problem is you need to run second query several times. Union may help in this case.
For MySQl you can use
RAND()
SELECT column FROM table
ORDER BY RAND()
LIMIT 4
I have got table with 300 000 rows. There is specially dedicated field (called order_number) in this table to story the number, which is later used to present the data from this table ordered by order_number field. What is the best and easy way to assign the random number/hash for each of the records in order to select the records ordered by this numbers? The number of rows in the table is not stable and may grow to 1 000 000, so the rand method should take it into the account.
Look at this tutorial on selecting random rows from a table.
If you don't want to use MySQL's built in RAND() function you could use something like this:
select max(id) from table;
$random_number = ...
select * from table where id > $random_number;
That should be a lot quicker.
UPDATE table SET order_number = sha2(id)
or
UPDATE table SET order_number = RAND(id)
sha2() is more random than RAND().
I know you've got enough answers but I would tell you how we did it in our company.
The first approach we use is with additional column for storing random number generated for each record/row. We have INDEX on this column, allowing us to order records by it.
id, name , ordering
1 , zlovic , 14
2 , silvas , 8
3 , jouzel , 59
SELECT * FROM table ORDER BY ordering ASC/DESC
POS: you have index and ordering is very fast
CONS: you will depend on new records to keep the randomization of the records
Second approach we have used is what Karl Roos gave an his answer. We retrieve the number of records in our database and using the > (greater) and some math we retrieve rows randomized. We are working with binary ids thus we need to keep autoincrement column to avoid random writings in InnoDB, sometimes we perform two or more queries to retrieve all of the data, keeping it randomized enough. (If you need 30 random items from 1,000,000 records you can run 3 simple SELECTs each for 10 items with different offset)
Hope this helps you. :)
To randomly select records from one table; do I have to always set a temporary variable in PHP? I need some help with selecting random rows within a CodeIgniter model, and then display three different ones in a view every time my homepage is viewed. Does anyone have any thoughts on how to solve this issue? Thanks in advance!
If you don't have a ton of rows, you can simply:
SELECT * FROM myTable ORDER BY RAND() LIMIT 3;
If you have many rows, this will get slow, but for smaller data sets it will work fine.
As Steve Michel mentions in his answer, this method can get very ugly for large tables. His suggestion is a good place to jump off from. If you know the approximate maximum integer PK on the table, you can do something like generating a random number between one and your max PK value, then grab random rows one at a time like:
$q="SELECT * FROM table WHERE id >= {$myRandomValue}";
$row = $db->fetchOne($q); //or whatever CI's interface to grab a single is like
Of course, if you need 3 random rows, you'll have three queries here, but as they're entirely on the PK, they'll be fast(er than randomizing the whole table).
I would do something like:
SELECT * FROM table ORDER BY RAND() LIMIT 1;
This will put the data in a random order and then return only the first row from that random order.
I have this piece of code in production to get a random quote. Using MySQL's RAND function was super slow. Even with 100 quotes in the database, I was noticing a lag time on the website. With this, there was no lag at all.
$result = mysql_query('SELECT COUNT(*) FROM quotes');
$count = mysql_fetch_row($result);
$id = rand(1, $count[0]);
$result = mysql_query("SELECT author, quote FROM quotes WHERE id=$id");
you need a query like this:
SELECT *
FROM tablename
WHERE somefield='something'
ORDER BY RAND() LIMIT 3
It is taken from the second result of
http://www.google.com/search?q=mysql+random
and it should work ;)
Ordering a big table by rand() can be very expensive if the table is very large. MySQL will need to build a temporary table and sort it. If you have primary key and you know how many rows are in the table, use LIMIT x,1 to grab a random row, where x is the number of the row you want to get.