Related
Please take a look at the following table:
I am building a search engine which returns card_id values, based on search of category_id and value_id values.
To better explain the search mechanism, imagine that we are trying to find a car (card_id) by supplying information what part (value_id) the car should has in every category (category_id).
In example, we may want to find a car (card_id), where category "Fuel Type" (category_id) has a value "Diesel" (value_id), and category "Gearbox" (category_id) has a value "Manual" (value_id).
My problem is that my knowledge is not sufficient to build a query, which will returns card_ids which contains more than one pair of category_id and value_id.
For example, if I want to search a car with diesel engine, I could build a query like this:
SELECT card_id FROM cars WHERE category_id=1 AND value_id=2
where category_id = 1 is a category "Fuel Type" and value_id = 2 is "Diesel".
My question is, how can I build a query, which will look for more category-value pairs? For example, I want to look for diesel cars with manual gearbox.
Any help will be very appreciated. Thank you in advance.
You can do this using aggregation and a having clause:
SELECT card_id
FROM cars
GROUP BY card_id
HAVING SUM(category_id = 1 AND value_id = 2) > 0 AND
SUM(category_id = 3 and value_id = 43) > 0;
Each condition in the having clause counts the number of rows that match a given condition. You can add as many conditions as you like. The first, for instance, says that there is at least one row where the category is 1 and the value is 2.
SQL Fiddle
Another approach is to create a user defined function that takes a table of attribute/value pairs and returns a table of matching cars. This has the advantage of allowing an arbitrary number of attribute/value pairs without resorting to dynamic SQL.
--Declare a "sample" table for proof of concept, replace this with your real data table
DECLARE #T TABLE(PID int, Attr Int, Val int)
--Populate the data table
INSERT INTO #T(PID , Attr , Val) VALUES (1,1,1), (1,3,5),(1,7,9),(2,1,2),(2,3,5),(2,7,9),(3,1,1),(3,3,5), (3,7,9)
--Declare this as a User Defined Table Type, the function would take this as an input
DECLARE #C TABLE(Attr Int, Val int)
--This would be populated by the code that calls the function
INSERT INTO #C (Attr , Val) VALUES (1,1),(7,9)
--The function (or stored procedure) body begins here
--Get a list of IDs for which there is not a requested attribute that doesn't have a matching value for that ID
SELECT DISTINCT PID
FROM #T as T
WHERE NOT EXISTS (SELECT C.ATTR FROM #C as C
WHERE NOT EXISTS (SELECT * FROM #T as I
WHERE I.Attr = C.Attr and I.Val = C.Val and I.PID = T.PID ))
I got two MySQL working fine and i'm trying to find a way to combine them into one single query.
First, it selects ID of an employee.
SELECT 'ID' FROM `employee` ORDER BY ID DESC LIMIT 1;
Let's say it returns ID 100;
Then update data of employees whose ID is 100
UPDATE 'LOG' SET `TIME_EXIT`='2013/02/22' WHERE `ID`='100';
Can i do it all in a single query?
Just add them together:
UPDATE LOG SET TIME_EXIT = '2013/02/22'
WHERE ID = (
SELECT ID
FROM employee
ORDER BY ID DESC
LIMIT
);
But based on that code currently it'll only ever update the last employee, you will need to select the correct employee by using some other identifier to ensure you have the correct one.
UPDATE LOG SET TIME_EXIT = '2013/02/22'
WHERE ID = (
SELECT ID
FROM employee
WHERE NAME = 'JOHN SMITH'
ORDER BY ID DESC
LIMIT 1
);
It's now a few months old, but maybe helps you or others finding this via google…
If you want to UPDATE a field in the same selected table use this:
UPDATE LOG SET
TIME_EXIT = '2013/02/22'
WHERE ID = (
SELECT ID
FROM (
SELECT ID
FROM LOG
WHERE whatEverYouWantToCheck = whateverYouNeed
) AS innerResult
)
So, you SELECT id from a subselect. If you try to subselect it directly, mySQL quites with your error message You can't specify target table 'log' for update in FROM clause, but this way you hide your subsubquery in a subquery and that seems to be fine. Don't forget the AS innerResult to avoid getting the error message #1248 - Every derived table must have its own alias. Also match the subsubquery field name to the subquery field name in case you do something like SELECT COUNT(*) or SELECT CONCAT('#', ID)
I'd like to use a single SQL query (in MySQL) to find the record which comes after one that I specify.
I.e., if the table has:
id, fruit
-- -----
1 apples
2 pears
3 oranges
I'd like to be able to do a query like:
SELECT * FROM table where previous_record has id=1 order by id;
(clearly that's not real SQL syntax, I'm just using pseudo-SQL to illustrate what I'm trying to achieve)
which would return:
2, pears
My current solution is just to fetch all the records, and look through them in PHP, but that's slower than I'd like. Is there a quicker way to do it?
I'd be happy with something that returned two rows -- i.e. the one with the specified value and the following row.
EDIT: Sorry, my question was badly worded. Unfortunately, my definition of "next" is not based on ID, but on alphabetical order of fruit name. Hence, my example above is wrong, and should return oranges, as it comes alphabetically next after apples. Is there a way to do the comparison on strings instead of ids?
After the question's edit and the simplification below, we can change it to
SELECT id FROM table WHERE fruit > 'apples' ORDER BY fruit LIMIT 1
SELECT * FROM table WHERE id > 1 ORDER BY id LIMIT 1
Even simpler
UPDATE:
SELECT * FROM table WHERE fruit > 'apples' ORDER BY fruit LIMIT 1
So simple, and no gymnastics required
Select * from Table
where id =
(Select Max(id) from Table
where id < #Id)
or, based on the string #fruitName = 'apples', or 'oranges' etc...
Select * from Table
where id =
(Select Max(id) from Table
where id < (Select id from Table
Where fruit = #fruitName))
I'm not familiar with the MySQL syntax, but with SQL Server you can do something with "top", for example:
SELECT TOP 1 * FROM table WHERE id > 1 ORDER BY id;
This assumes that the id field is unique. If it is not unique (say, a foreign key), you can do something similar and then join back against the same table.
Since I don't use MySQL, I am not sure of the syntax, but would imagine it to be similar.
Unless you specify a sort order, I don't believe the concepts of "previous" or "next" are available to you in SQL. You aren't guaranteed a particular order by the RDBMS by default. If you can sort by some column into ascending or descending order that's another matter.
This should work. The string 'apples' will need to be a parameter.
Fill in that parameter with a string, and this query will return the entire record for the first fruit after that item, in alphabetical order.
Unlike the LIMIT 1 approach, this should be platform-independent.
--STEP THREE: Get the full record w/the ID we found in step 2
select *
from
fruits fr
,(
--STEP TWO: Get the ID # of the name we found in step 1
select
min(vendor_id) min_id
from
fruits fr1
,(
--STEP ONE: Get the next name after "apples"
select min(name) next_name
from fruits frx
where frx.name > 'apples'
) minval
where fr1.name = minval.next_name
) x
where fr.vendor_id = x.min_id;
The equivalent to the LIMIT 1 approach in Oracle (just for reference) would be this:
select *
from
(
select *
from fruits frx
where frx.name > 'apples'
order by name
)
where rownum = 1
I don't know MySQL SQL but I still try
select n.id
from fruit n
, fruit p
where n.id = p.id + 1;
edit:
select n.id, n.fruitname
from fruits n
, fruits p
where n.id = p.id + 1;
edit two:
Jason Lepack has said that that doesn't work when there are gaps and that is true and I should read the question better.
I should have used analytics to sort the results on fruitname
select id
, fruitname
, lead(id) over (order by fruitname) id_next
, lead(fruitname) over (order by fruitname) fruitname_next
from fruits;
If you are using MS SQL Server 2008 (not sure if available for previous versions)...
In the event that you are trying to find the next record and you do not have a unique ID to reference in an applicable manner, try using ROW_NUMBER(). See this link
Depending on how savvy your T-SQL skill is, you can create row numbers based on your sorting order. Then you can find more than just the previous and next record. Utilize it in views or sub-queries to find another record relative to the current record's row number.
SELECT cur.id as id, nxt.id as nextId, prev.id as prevId FROM video as cur
LEFT JOIN video as nxt ON nxt.id > cur.id
LEFT JOIN video as prev ON prev.id < cur.id
WHERE cur.id = 12
ORDER BY prev.id DESC, nxt.id ASC
LIMIT 1
If you want the item with previous and next item this query lets you do just that.
This also allows You to have gaps in the data!
How about this:
Select * from table where id = 1 + 1
What is a fast way to select a random row from a large mysql table?
I'm working in php, but I'm interested in any solution even if it's in another language.
Grab all the id's, pick a random one from it, and retrieve the full row.
If you know the id's are sequential without holes, you can just grab the max and calculate a random id.
If there are holes here and there but mostly sequential values, and you don't care about a slightly skewed randomness, grab the max value, calculate an id, and select the first row with an id equal to or above the one you calculated. The reason for the skewing is that id's following such holes will have a higher chance of being picked than ones that follow another id.
If you order by random, you're going to have a terrible table-scan on your hands, and the word quick doesn't apply to such a solution.
Don't do that, nor should you order by a GUID, it has the same problem.
I knew there had to be a way to do it in a single query in a fast way. And here it is:
A fast way without involvement of external code, kudos to
http://jan.kneschke.de/projects/mysql/order-by-rand/
SELECT name
FROM random AS r1 JOIN
(SELECT (RAND() *
(SELECT MAX(id)
FROM random)) AS id)
AS r2
WHERE r1.id >= r2.id
ORDER BY r1.id ASC
LIMIT 1;
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.)
This approach can cause a problem if the resulting numbers are badly distributed; IIRC, this has been fixed on MediaWiki, so if you decide to do it this way you should take a look at the code to see how it's currently done (probably they periodically regenerate the random number column).
Here's a solution that runs fairly quickly, and it gets a better random distribution without depending on id values being contiguous or starting at 1.
SET #r := (SELECT ROUND(RAND() * (SELECT COUNT(*) FROM mytable)));
SET #sql := CONCAT('SELECT * FROM mytable LIMIT ', #r, ', 1');
PREPARE stmt1 FROM #sql;
EXECUTE stmt1;
Maybe you could do something like:
SELECT * FROM table
WHERE id=
(FLOOR(RAND() *
(SELECT COUNT(*) FROM table)
)
);
This is assuming your ID numbers are all sequential with no gaps.
Add a column containing a calculated random value to each row, and use that in the ordering clause, limiting to one result upon selection. This works out faster than having the table scan that ORDER BY RANDOM() causes.
Update: You still need to calculate some random value prior to issuing the SELECT statement upon retrieval, of course, e.g.
SELECT * FROM `foo` WHERE `foo_rand` >= {some random value} LIMIT 1
There is another way to produce random rows using only a query and without order by rand().
It involves User Defined Variables.
See how to produce random rows from a table
In order to find random rows from a table, don’t use ORDER BY RAND() because it forces MySQL to do a full file sort and only then to retrieve the limit rows number required. In order to avoid this full file sort, use the RAND() function only at the where clause. It will stop as soon as it reaches to the required number of rows.
See
http://www.rndblog.com/how-to-select-random-rows-in-mysql/
if you don't delete row in this table, the most efficient way is:
(if you know the mininum id just skip it)
SELECT MIN(id) AS minId, MAX(id) AS maxId FROM table WHERE 1
$randId=mt_rand((int)$row['minId'], (int)$row['maxId']);
SELECT id,name,... FROM table WHERE id=$randId LIMIT 1
I see here a lot of solution. One or two seems ok but other solutions have some constraints. But the following solution will work for all situation
select a.* from random_data a, (select max(id)*rand() randid from random_data) b
where a.id >= b.randid limit 1;
Here, id, don't need to be sequential. It could be any primary key/unique/auto increment column. Please see the following Fastest way to select a random row from a big MySQL table
Thanks
Zillur
- www.techinfobest.com
For selecting multiple random rows from a given table (say 'words'), our team came up with this beauty:
SELECT * FROM
`words` AS r1 JOIN
(SELECT MAX(`WordID`) as wid_c FROM `words`) as tmp1
WHERE r1.WordID >= (SELECT (RAND() * tmp1.wid_c) AS id) LIMIT n
The classic "SELECT id FROM table ORDER BY RAND() LIMIT 1" is actually OK.
See the follow excerpt from the MySQL manual:
If you use LIMIT row_count with ORDER BY, MySQL ends the sorting as soon as it has found the first row_count rows of the sorted result, rather than sorting the entire result.
With a order yo will do a full scan table.
Its best if you do a select count(*) and later get a random row=rownum between 0 and the last registry
An easy but slow way would be (good for smallish tables)
SELECT * from TABLE order by RAND() LIMIT 1
In pseudo code:
sql "select id from table"
store result in list
n = random(size of list)
sql "select * from table where id=" + list[n]
This assumes that id is a unique (primary) key.
Take a look at this link by Jan Kneschke or this SO answer as they both discuss the same question. The SO answer goes over various options also and has some good suggestions depending on your needs. Jan goes over all the various options and the performance characteristics of each. He ends up with the following for the most optimized method by which to do this within a MySQL select:
SELECT name
FROM random AS r1 JOIN
(SELECT (RAND() *
(SELECT MAX(id)
FROM random)) AS id)
AS r2
WHERE r1.id >= r2.id
ORDER BY r1.id ASC
LIMIT 1;
HTH,
-Dipin
I'm a bit new to SQL but how about generating a random number in PHP and using
SELECT * FROM the_table WHERE primary_key >= $randNr
this doesn't solve the problem with holes in the table.
But here's a twist on lassevks suggestion:
SELECT primary_key FROM the_table
Use mysql_num_rows() in PHP create a random number based on the above result:
SELECT * FROM the_table WHERE primary_key = rand_number
On a side note just how slow is SELECT * FROM the_table:
Creating a random number based on mysql_num_rows() and then moving the data pointer to that point mysql_data_seek(). Just how slow will this be on large tables with say a million rows?
I ran into the problem where my IDs were not sequential. What I came up with this.
SELECT * FROM products WHERE RAND()<=(5/(SELECT COUNT(*) FROM products)) LIMIT 1
The rows returned are approximately 5, but I limit it to 1.
If you want to add another WHERE clause it becomes a bit more interesting. Say you want to search for products on discount.
SELECT * FROM products WHERE RAND()<=(100/(SELECT COUNT(*) FROM pt_products)) AND discount<.2 LIMIT 1
What you have to do is make sure you are returning enough result which is why I have it set to 100. Having a WHERE discount<.2 clause in the subquery was 10x slower, so it's better to return more results and limit.
Use the below query to get the random row
SELECT user_firstname ,
COUNT(DISTINCT usr_fk_id) cnt
FROM userdetails
GROUP BY usr_fk_id
ORDER BY cnt ASC
LIMIT 1
In my case my table has an id as primary key, auto-increment with no gaps, so I can use COUNT(*) or MAX(id) to get the number of rows.
I made this script to test the fastest operation:
logTime();
query("SELECT COUNT(id) FROM tbl");
logTime();
query("SELECT MAX(id) FROM tbl");
logTime();
query("SELECT id FROM tbl ORDER BY id DESC LIMIT 1");
logTime();
The results are:
Count: 36.8418693542479 ms
Max: 0.241041183472 ms
Order: 0.216960906982 ms
Answer with the order method:
SELECT FLOOR(RAND() * (
SELECT id FROM tbl ORDER BY id DESC LIMIT 1
)) n FROM tbl LIMIT 1
...
SELECT * FROM tbl WHERE id = $result;
I have used this and the job was done
the reference from here
SELECT * FROM myTable WHERE RAND()<(SELECT ((30/COUNT(*))*10) FROM myTable) ORDER BY RAND() LIMIT 30;
Create a Function to do this most likely the best answer and most fastest answer here!
Pros - Works even with Gaps and extremely fast.
<?
$sqlConnect = mysqli_connect('localhost','username','password','database');
function rando($data,$find,$max = '0'){
global $sqlConnect; // Set as mysqli connection variable, fetches variable outside of function set as GLOBAL
if($data == 's1'){
$query = mysqli_query($sqlConnect, "SELECT * FROM `yourtable` ORDER BY `id` DESC LIMIT {$find},1");
$fetched_data = mysqli_fetch_assoc($query);
if(mysqli_num_rows($fetched_data>0){
return $fetch_$data;
}else{
rando('','',$max); // Start Over the results returned nothing
}
}else{
if($max != '0'){
$irand = rand(0,$max);
rando('s1',$irand,$max); // Start rando with new random ID to fetch
}else{
$query = mysqli_query($sqlConnect, "SELECT `id` FROM `yourtable` ORDER BY `id` DESC LIMIT 0,1");
$fetched_data = mysqli_fetch_assoc($query);
$max = $fetched_data['id'];
$irand = rand(1,$max);
rando('s1',$irand,$max); // Runs rando against the random ID we have selected if data exist will return
}
}
}
$your_data = rando(); // Returns listing data for a random entry as a ASSOC ARRAY
?>
Please keep in mind this code as not been tested but is a working concept to return random entries even with gaps.. As long as the gaps are not huge enough to cause a load time issue.
Quick and dirty method:
SET #COUNTER=SELECT COUNT(*) FROM your_table;
SELECT PrimaryKey
FROM your_table
LIMIT 1 OFFSET (RAND() * #COUNTER);
The complexity of the first query is O(1) for MyISAM tables.
The second query accompanies a table full scan. Complexity = O(n)
Dirty and quick method:
Keep a separate table for this purpose only. You should also insert the same rows to this table whenever inserting to the original table. Assumption: No DELETEs.
CREATE TABLE Aux(
MyPK INT AUTO_INCREMENT,
PrimaryKey INT
);
SET #MaxPK = (SELECT MAX(MyPK) FROM Aux);
SET #RandPK = CAST(RANDOM() * #MaxPK, INT)
SET #PrimaryKey = (SELECT PrimaryKey FROM Aux WHERE MyPK = #RandPK);
If DELETEs are allowed,
SET #delta = CAST(#RandPK/10, INT);
SET #PrimaryKey = (SELECT PrimaryKey
FROM Aux
WHERE MyPK BETWEEN #RandPK - #delta AND #RandPK + #delta
LIMIT 1);
The overall complexity is O(1).
SELECT DISTINCT * FROM yourTable WHERE 4 = 4 LIMIT 1;
What is a fast way to select a random row from a large mysql table?
I'm working in php, but I'm interested in any solution even if it's in another language.
Grab all the id's, pick a random one from it, and retrieve the full row.
If you know the id's are sequential without holes, you can just grab the max and calculate a random id.
If there are holes here and there but mostly sequential values, and you don't care about a slightly skewed randomness, grab the max value, calculate an id, and select the first row with an id equal to or above the one you calculated. The reason for the skewing is that id's following such holes will have a higher chance of being picked than ones that follow another id.
If you order by random, you're going to have a terrible table-scan on your hands, and the word quick doesn't apply to such a solution.
Don't do that, nor should you order by a GUID, it has the same problem.
I knew there had to be a way to do it in a single query in a fast way. And here it is:
A fast way without involvement of external code, kudos to
http://jan.kneschke.de/projects/mysql/order-by-rand/
SELECT name
FROM random AS r1 JOIN
(SELECT (RAND() *
(SELECT MAX(id)
FROM random)) AS id)
AS r2
WHERE r1.id >= r2.id
ORDER BY r1.id ASC
LIMIT 1;
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.)
This approach can cause a problem if the resulting numbers are badly distributed; IIRC, this has been fixed on MediaWiki, so if you decide to do it this way you should take a look at the code to see how it's currently done (probably they periodically regenerate the random number column).
Here's a solution that runs fairly quickly, and it gets a better random distribution without depending on id values being contiguous or starting at 1.
SET #r := (SELECT ROUND(RAND() * (SELECT COUNT(*) FROM mytable)));
SET #sql := CONCAT('SELECT * FROM mytable LIMIT ', #r, ', 1');
PREPARE stmt1 FROM #sql;
EXECUTE stmt1;
Maybe you could do something like:
SELECT * FROM table
WHERE id=
(FLOOR(RAND() *
(SELECT COUNT(*) FROM table)
)
);
This is assuming your ID numbers are all sequential with no gaps.
Add a column containing a calculated random value to each row, and use that in the ordering clause, limiting to one result upon selection. This works out faster than having the table scan that ORDER BY RANDOM() causes.
Update: You still need to calculate some random value prior to issuing the SELECT statement upon retrieval, of course, e.g.
SELECT * FROM `foo` WHERE `foo_rand` >= {some random value} LIMIT 1
There is another way to produce random rows using only a query and without order by rand().
It involves User Defined Variables.
See how to produce random rows from a table
In order to find random rows from a table, don’t use ORDER BY RAND() because it forces MySQL to do a full file sort and only then to retrieve the limit rows number required. In order to avoid this full file sort, use the RAND() function only at the where clause. It will stop as soon as it reaches to the required number of rows.
See
http://www.rndblog.com/how-to-select-random-rows-in-mysql/
if you don't delete row in this table, the most efficient way is:
(if you know the mininum id just skip it)
SELECT MIN(id) AS minId, MAX(id) AS maxId FROM table WHERE 1
$randId=mt_rand((int)$row['minId'], (int)$row['maxId']);
SELECT id,name,... FROM table WHERE id=$randId LIMIT 1
I see here a lot of solution. One or two seems ok but other solutions have some constraints. But the following solution will work for all situation
select a.* from random_data a, (select max(id)*rand() randid from random_data) b
where a.id >= b.randid limit 1;
Here, id, don't need to be sequential. It could be any primary key/unique/auto increment column. Please see the following Fastest way to select a random row from a big MySQL table
Thanks
Zillur
- www.techinfobest.com
For selecting multiple random rows from a given table (say 'words'), our team came up with this beauty:
SELECT * FROM
`words` AS r1 JOIN
(SELECT MAX(`WordID`) as wid_c FROM `words`) as tmp1
WHERE r1.WordID >= (SELECT (RAND() * tmp1.wid_c) AS id) LIMIT n
The classic "SELECT id FROM table ORDER BY RAND() LIMIT 1" is actually OK.
See the follow excerpt from the MySQL manual:
If you use LIMIT row_count with ORDER BY, MySQL ends the sorting as soon as it has found the first row_count rows of the sorted result, rather than sorting the entire result.
With a order yo will do a full scan table.
Its best if you do a select count(*) and later get a random row=rownum between 0 and the last registry
An easy but slow way would be (good for smallish tables)
SELECT * from TABLE order by RAND() LIMIT 1
In pseudo code:
sql "select id from table"
store result in list
n = random(size of list)
sql "select * from table where id=" + list[n]
This assumes that id is a unique (primary) key.
Take a look at this link by Jan Kneschke or this SO answer as they both discuss the same question. The SO answer goes over various options also and has some good suggestions depending on your needs. Jan goes over all the various options and the performance characteristics of each. He ends up with the following for the most optimized method by which to do this within a MySQL select:
SELECT name
FROM random AS r1 JOIN
(SELECT (RAND() *
(SELECT MAX(id)
FROM random)) AS id)
AS r2
WHERE r1.id >= r2.id
ORDER BY r1.id ASC
LIMIT 1;
HTH,
-Dipin
I'm a bit new to SQL but how about generating a random number in PHP and using
SELECT * FROM the_table WHERE primary_key >= $randNr
this doesn't solve the problem with holes in the table.
But here's a twist on lassevks suggestion:
SELECT primary_key FROM the_table
Use mysql_num_rows() in PHP create a random number based on the above result:
SELECT * FROM the_table WHERE primary_key = rand_number
On a side note just how slow is SELECT * FROM the_table:
Creating a random number based on mysql_num_rows() and then moving the data pointer to that point mysql_data_seek(). Just how slow will this be on large tables with say a million rows?
I ran into the problem where my IDs were not sequential. What I came up with this.
SELECT * FROM products WHERE RAND()<=(5/(SELECT COUNT(*) FROM products)) LIMIT 1
The rows returned are approximately 5, but I limit it to 1.
If you want to add another WHERE clause it becomes a bit more interesting. Say you want to search for products on discount.
SELECT * FROM products WHERE RAND()<=(100/(SELECT COUNT(*) FROM pt_products)) AND discount<.2 LIMIT 1
What you have to do is make sure you are returning enough result which is why I have it set to 100. Having a WHERE discount<.2 clause in the subquery was 10x slower, so it's better to return more results and limit.
Use the below query to get the random row
SELECT user_firstname ,
COUNT(DISTINCT usr_fk_id) cnt
FROM userdetails
GROUP BY usr_fk_id
ORDER BY cnt ASC
LIMIT 1
In my case my table has an id as primary key, auto-increment with no gaps, so I can use COUNT(*) or MAX(id) to get the number of rows.
I made this script to test the fastest operation:
logTime();
query("SELECT COUNT(id) FROM tbl");
logTime();
query("SELECT MAX(id) FROM tbl");
logTime();
query("SELECT id FROM tbl ORDER BY id DESC LIMIT 1");
logTime();
The results are:
Count: 36.8418693542479 ms
Max: 0.241041183472 ms
Order: 0.216960906982 ms
Answer with the order method:
SELECT FLOOR(RAND() * (
SELECT id FROM tbl ORDER BY id DESC LIMIT 1
)) n FROM tbl LIMIT 1
...
SELECT * FROM tbl WHERE id = $result;
I have used this and the job was done
the reference from here
SELECT * FROM myTable WHERE RAND()<(SELECT ((30/COUNT(*))*10) FROM myTable) ORDER BY RAND() LIMIT 30;
Create a Function to do this most likely the best answer and most fastest answer here!
Pros - Works even with Gaps and extremely fast.
<?
$sqlConnect = mysqli_connect('localhost','username','password','database');
function rando($data,$find,$max = '0'){
global $sqlConnect; // Set as mysqli connection variable, fetches variable outside of function set as GLOBAL
if($data == 's1'){
$query = mysqli_query($sqlConnect, "SELECT * FROM `yourtable` ORDER BY `id` DESC LIMIT {$find},1");
$fetched_data = mysqli_fetch_assoc($query);
if(mysqli_num_rows($fetched_data>0){
return $fetch_$data;
}else{
rando('','',$max); // Start Over the results returned nothing
}
}else{
if($max != '0'){
$irand = rand(0,$max);
rando('s1',$irand,$max); // Start rando with new random ID to fetch
}else{
$query = mysqli_query($sqlConnect, "SELECT `id` FROM `yourtable` ORDER BY `id` DESC LIMIT 0,1");
$fetched_data = mysqli_fetch_assoc($query);
$max = $fetched_data['id'];
$irand = rand(1,$max);
rando('s1',$irand,$max); // Runs rando against the random ID we have selected if data exist will return
}
}
}
$your_data = rando(); // Returns listing data for a random entry as a ASSOC ARRAY
?>
Please keep in mind this code as not been tested but is a working concept to return random entries even with gaps.. As long as the gaps are not huge enough to cause a load time issue.
Quick and dirty method:
SET #COUNTER=SELECT COUNT(*) FROM your_table;
SELECT PrimaryKey
FROM your_table
LIMIT 1 OFFSET (RAND() * #COUNTER);
The complexity of the first query is O(1) for MyISAM tables.
The second query accompanies a table full scan. Complexity = O(n)
Dirty and quick method:
Keep a separate table for this purpose only. You should also insert the same rows to this table whenever inserting to the original table. Assumption: No DELETEs.
CREATE TABLE Aux(
MyPK INT AUTO_INCREMENT,
PrimaryKey INT
);
SET #MaxPK = (SELECT MAX(MyPK) FROM Aux);
SET #RandPK = CAST(RANDOM() * #MaxPK, INT)
SET #PrimaryKey = (SELECT PrimaryKey FROM Aux WHERE MyPK = #RandPK);
If DELETEs are allowed,
SET #delta = CAST(#RandPK/10, INT);
SET #PrimaryKey = (SELECT PrimaryKey
FROM Aux
WHERE MyPK BETWEEN #RandPK - #delta AND #RandPK + #delta
LIMIT 1);
The overall complexity is O(1).
SELECT DISTINCT * FROM yourTable WHERE 4 = 4 LIMIT 1;