mysql - SQL to get even distribution from n groups - fetch random items -

-

i have following tables:

table product id int(11) title varchar(400)  table tag id int(11) text varchar(100)  table product_tag_map product_id int(11) tag_id int(11)

product_tag_map maps tags product. distribution of tags in system isn't normal, i.e., tags have more products others.

i'm trying write sql fetch 25 random products: 5 products per tag, 5 tags (so that's 5x5 = 25).

found answer here: how can distribution using id in(1,2,3,4)

but doesn't yield random products - fetches same products per tag.

here sql have:

set @last_tag = 0; set @count_tag = 0;  select distinct id ( select   product.*,   @count_tag := if(@last_tag = product_tag_map.tag_id, @count_tag, 0) + 1 tag_row_number,   @last_tag := product_tag_map.tag_id product   left join product_tag_map on (product_tag_map.product_id=product.id)   product_tag_map.tag_id in (245,255,259,281,296) ) subquery tag_row_number <= 5;

how make return random products per tag?

any appreciated! thanks.

there lot of tricks in query :

  1. add level of nesting use limit in subquery : mysql subquery limit
  2. add row_number functionality mysql : how select first/least/max row per group in sql

the final result lot of subquery:

select tag.name, t0.id mapid (     select *           , @num := if(@type = tag_id, @num + 1, 1) row_number          , @type := tag_id dummy     (         select *         map m         tag_id in         (             select *                         (                 select id                 tag                 order rand() limit 5             ) t         )      order tag_id, rand()     ) maintable   , (select @num:=0) foo   , (select @type:=0) foo2 ) t0      inner join tag         on t0.tag_id = tag.id row_number <= 5

sql fiddle

the idea select first 5 random tags. not difficult, simple order rand() limit 5.

then tricky part simulate row_number() over(partition tag_id, rand()), because ranking each item randomly, partition tag need. declare variable , query show.

finally, filter row_number, , have 25 random items!


Popular posts from this blog

How to calculate SNR of signals in MATLAB? -

-
i've encountered simple, yet fundamental problem on calculating snr: there several signals : s1 = original , pure clean signal without noise. n1 = white gaussian noise going added s1. s2 = s1 + n1 (the noisy signal before performing noise-reduction algorithm) s3 = noise-reduced signal (after performing noise reduction algorithm) n2 = s3 - s1 (the amount of noise after performing noise-reduction algorithm) now want compare snr before , after performing noise-reduction algorithm. which signals should consider snr_before!? s1/n1 or s2/n1 ? which signals should consider snr_after!? s3/n1 or s3/n2 ? which commands or functions should use in matlab in order compute snr_before , snr_after? thanks billion putting time on helping me. what know calculating snr before : snrbeforenoise = mean( signal .^ 2 ) / mean( noise .^ 2 ); and snr after : residual_noise = signal - noise_reduced_signal; snr_after = mean( signal .^ 2 ) / mean( residual_noise .^ 2
Read more

c# - Attempting to upload to FTP: System.Net.WebException: System error -

-
i have api takes in xml , uploads files based on information in xml. uploads on schedule (also xml), , have tested surrounding , know works. i getting error 40% of time on first file attempt upload in each time cycle (time cycle = 45 minutes files, 30 minutes others). here code upload: try { loggerftp.log("uploading file: " + filename, false); // create request. ftpwebrequest request = (ftpwebrequest)webrequest.create(appsettingsftp.ftpurl + @"/" + filename); request.method = webrequestmethods.ftp.uploadfile; request.timeout = 6000000; //set 100 minutes //request.timeout = -1; //set infinite // add login credentials. request.credentials = new networkcredential(appsettingsftp.ftplogin, appsettingsftp.ftppassword); // grab file contents. streamreader sourcestream = new streamreader(appsettingsftp.uploadfiledirectory + filename); byte[] filecontents = encoding.utf8.getbytes(sourcestream.readtoend()); sourcest
Read more

ios - UISlider customization: how to properly add shadow to custom knob image -

-
Image
i need place custom knob image shadow uislider. far have image of 38x38, of there region defines shadow of knob. now while using image, when slide knob extreme ends actual knob not able cover end points, hence end points visible under transparent region of knob image this code using customize uislider uiimage *slidebtimg = [uiimage imagenamed:sliderknobimg]; [self.ratingslider setthumbimage:slidebtimg forstate:uicontrolstatenormal]; [self.ratingslider setthumbimage:slidebtimg forstate:uicontrolstatehighlighted]; how can implement custom knob shadow beneath it, covering end points?? look inside uislider.h , there methods let customize slider knob , track needed. (ios 2.0 , above) // lets subclass lay out track , thumb needed - (cgrect)minimumvalueimagerectforbounds:(cgrect)bounds; - (cgrect)maximumvalueimagerectforbounds:(cgrect)bounds; - (cgrect)trackrectforbounds:(cgrect)bounds; - (cgrect)thumbrectforbounds:(cgrect)bounds trackrect:(cgrect)rect value:
Read more