sql - mysql weighted random results - how to get changed variable value after SELECT -

i have table keywords columns keyword , weight. goal randomly select 1 keyword, regard weight (probability). found 2 ways solve this, latter 1 more elegant (and consumes less ressources) - dont run. see yourself.

the table , records:

create table if not exists `keywords` (   `keyword` varchar(100) collate utf8_bin not null,   `weight` int(11) not null,   unique key `keywords` (`keyword`),   key `rate` (`weight`) ) engine=myisam default charset=utf8 collate=utf8_bin;  insert `keywords` (`keyword`, `weight`) values ('google', 50), ('microsoft', 20), ('apple', 10), ('yahoo', 5), ('bing', 5), ('xing', 5), ('cool', 5); 

query 1

consumes more ressources, work on 5k+ records. source why mysql query using rand() return no results third of time?:

select *  `keywords` order -log(1.0 - rand()) / weight limit 1 

query 2

sums weights @weight_sum. sets @weight_point rand() number within range. loops through records, substracting weight @weight_pos , setting @keyword current keywords.keyword. until @weight_pos < 0. keeps keyword. source random weighted choice in t-sql

set @keyword = 0; set @weight_sum = (select sum(weight) keywords); set @rand = rand(); set @weight_point = round(((@weight_sum - 1) * @rand + 1), 0); set @weight_pos = @weight_point;  select     keyword,     weight,      @keyword:=case         when @weight_pos < 0 @keyword         else keyword     end test,      (@weight_pos:=(@weight_pos - weight)) curr_weight,     @weight_point,     @keyword,     @weight_pos,     @rand,     @weight_sum     keywords; 

see phpmyadmin results here http://postimg.org/image/stgpd776f/

my question

how value in @keyword, or test column holds in end? adding select @keyword afterwards doesn't change anything.

ok, think question more or less basic mysql-question. achieved wanted encapsulating above select statement select, filtered first one's result searched. sorry bothering you. see query:

set @keyword = 0; set @weight_sum = (select sum(weight) keywords); set @rand = rand(); set @weight_point = round(((@weight_sum - 1) * @rand + 1), 0); set @weight_pos = @weight_point; select t.test ( select     keyword,     weight,     @keyword:=case         when @weight_pos < 0 @keyword         else keyword     end test,     (@weight_pos:=(@weight_pos - weight)) curr_weight,     @weight_point,     #@keyword,     @weight_pos,     @rand,     @weight_sum     keywords  ) t      t.curr_weight < 0 limit     1;