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. :)
Related
We have records with a count field on an unique id.
The columns are:
mainId = unique
mainIdCount = 1320 (this 'views' field gets a + 1 when the page is visited)
How can you insert all these mainIdCount's as seperate records in another table IN ANOTHER DBASE in one query?
Yes, I do mean 1320 times an insert with the same mainId! :-)
We actually have records that go over 10,000 times an id. It just has to be like this.
This is a weird one, but we do need the copies of all these (just) counts like this.
The most straightforward way to this is with a JOIN operation between your table, and another row source that provides a set of integers. We'd match each row from our original table to as many rows from the set of integer as needed to satisfy the desired result.
As a brief example of the pattern:
INSERT INTO newtable (mainId,n)
SELECT t.mainId
, r.n
FROM mytable t
JOIN ( SELECT 1 AS n
UNION ALL SELECT 2
UNION ALL SELECT 3
UNION ALL SELECT 4
UNION ALL SELECT 5
) r
WHERE r.n <= t.mainIdCount
If mytable contains row mainId=5 mainIdCount=4, we'd get back rows (5,1),(5,2),(5,3),(5,4)
Obviously, the rowsource r needs to be of sufficient size. The inline view I've demonstrated here would return a maximum of five rows. For larger sets, it would be beneficial to use a table rather than an inline view.
This leads to the followup question, "How do I generate a set of integers in MySQL",
e.g. Generating a range of numbers in MySQL
And getting that done is a bit tedious. We're looking forward to an eventual feature in MySQL that will make it much easier to return a bounded set of integer values; until then, having a pre-populated table is the most efficient approach.
Im facing a unique challenge.
I got a table with 100 numbers called HUNDREDNUMBERS.
I want to select the best quarter (75 to 100 numbers),
and place them into another table called BESTQUARTER.
I also want to select the worst quarter (1 to 25 numbers)
I want to place these into another table called WORSTQUARTER.
here's my Mysql code, so far,
$Extract_Data = "
CREATE TABLE $BESTQUARTER
SELECT
HUNDREDNUMBERS.number
FROM
HUNDREDNUMBERS order by
HUNDREDNUMBERS.number desc LIMIT 25 ";
$QuerySuccess = mysql_query($Extract_Data, $connection);
and for the other table....
$Extract_Data = "
CREATE TABLE $WORSTQUARTER
SELECT
HUNDREDNUMBERS.number
FROM
HUNDREDNUMBERS order by
HUNDREDNUMBERS.number asc LIMIT 25 ";
$QuerySuccess = mysql_query($Extract_Data, $connection);
The problem is that this script is not 100% correct every time.
Notice the ASC and the DESC in the two queries.
It's an ingenious way of trying to sort the numbers.
BTW, some of the numbers in the HUNDREDNUMBERS table have decimal points.
I need the data in the two new tables BESTQUARTER and WORSTQUARTER for further processing.
Any help is greatly appreciated
You're doing string comparisons and those follow different rules than numeric data types; I would suggest to change your sort expressions:
ORDER BY CAST(HUNDREDNUMBERS.number AS UNSIGNED) DESC|ASC
Instead of UNSIGNED you could also use SIGNED or DECIMAL(M, N) if you need to support negative numbers or floating points respectively.
Alternatively (and preferably), you could change the number column to a type that sorts properly by itself; VARCHAR should mostly be used for text.
You should check the data types. Make sure the the numbers are stored as at least a decimal. Other data types can cause the sorting to be off (and is a quite common mistake). It seems simple, but your code actually looks to be correct from what my understanding is of the question.
If you have only 100 numbers, I would suggest that you create a view with a rank, and use that for subsequent processing. Using intermediate tables seems like overkill:
select hn.*,
(select count(*) from hundrednumbers hn2 where hn2.number <= hn.number
) as rank
from HundredNumbers hn
With an index on hundrednumbers(number), this will even have decent performance.
It is possible that the problem you are encountering is duplicates in the original data. If so, looking at the ranks can help you figure out what to do in this situation.
After long hours of thinking and testing, i believe i finally cracked it.
1) I changed the fieldname "numbers" to DOUBLE UNSIGNED.
(initially i was using VARCHAR(50) )
2) Whenever you are using two or more tables that have the same field names, prefix EVERY fieldname with its tablename.
I did that and it worked, as you shall see in the full query below.
3) the original data had multiple occurrences of the same numbers,
ie there were several instances of rows with the value 100.
MySQL transferred only a single row with the value 100, into the table BESTQUARTER. (i don't know why).
uniqueid | id | numbers
1 200 100
2 6 100
3 76 100
4 64 99.009987655
5 10 95.98765432
6 11 11.98765432
7 12 25.12
8 13 53.173543
9 153 72.87676
10 32 99
So i added "GROUP By" and used the ID field.
(nb: "uniqueid" column is the primary key, "id" is a unique key that uniquely identifies each number)
Here's the new code
create table BESTQUATER
select
HUNDREDNUMBERS.uniqueid ,
HUNDREDNUMBERS.id,
HUNDREDNUMBERS.numbers
FROM
HUNDREDNUMBERS
group by HUNDREDNUMBERS.id
ORDER BY HUNDREDNUMBERS.numbers DESC LIMIT 25
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.
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
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.