大家好,星期天給大家。 我需要從每個組中選擇N個隨機記錄。每個羣組選擇N個隨機記錄
從Quassnoi
http://explainextended.com/2009/03/01/selecting-random-rows/的查詢
開始選擇X我寫這個存儲過程
delimiter //
drop procedure if exists casualiPerGruppo //
create procedure casualiPerGruppo(in tabella varchar(50),in campo varchar(50),in numPerGruppo int)
comment 'Selezione di N record casuali per gruppo'
begin
declare elenco_campi varchar(255);
declare valore int;
declare finite int default 0;
declare query1 varchar(250);
declare query2 varchar(250);
declare query3 varchar(250);
declare query4 varchar(250);
declare cur_gruppi cursor for select gruppo from tmp_view;
declare continue handler for not found set finite = 1;
drop table if exists tmp_casuali;
set @query1 = concat('create temporary table tmp_casuali like ', tabella);
prepare stmt from @query1;
execute stmt;
deallocate prepare stmt;
set @query2 = concat('create or replace view tmp_view as select ',campo,' as gruppo from ',tabella,' group by ',campo);
prepare stmt from @query2;
execute stmt;
deallocate prepare stmt;
open cur_gruppi;
mio_loop:loop
fetch cur_gruppi into valore;
if finite = 1 then
leave mio_loop;
end if;
set @query3 = concat("select group_concat(column_name) into @elenco_campi
from information_schema.columns
where table_name = '",tabella,"' and table_schema = database()");
prepare stmt from @query3;
execute stmt;
deallocate prepare stmt;
set @query4 = concat('insert into tmp_casuali select ',
@elenco_campi,' from (
select @cnt := count(*) + 1,
@lim :=', numPerGruppo,
' from ',tabella,
' where ',campo,' = ', valore,
') vars
straight_join
(
select r.*,
@lim := @lim - 1
from ', tabella, ' r
where (@cnt := @cnt - 1)
and rand() < @lim/@cnt and ', campo, ' = ', valore ,
') i');
prepare stmt from @query4;
execute stmt;
deallocate prepare stmt;
end loop;
close cur_gruppi;
select * from tmp_casuali;
end //
delimiter ;
,我以這種方式給你一個想法記得隨機記錄:
create table prova (
id int not null auto_increment primary key,
id_gruppo int,
altro varchar(10)
) engine = myisam;
insert into prova (id_gruppo,altro) values
(1,'aaa'),(2,'bbb'),(3,'ccc'),(1,'ddd'),(1,'eee'),(2,'fff'),
(2,'ggg'),(2,'hhh'),(3,'iii'),(3,'jjj'),(3,'kkk'),(1,'lll'),(4,'mmm');
call casualiPerGruppo('prova','id_gruppo',2);
我的問題是,Quassnoi查詢,甚至t霍夫非常高效,在大型賽馬比賽中需要1秒鐘的時間。所以如果我多次將它應用於我的sp,總時間會增加很多。
你能告訴我一個更好的方法來解決我的問題嗎? 在此先感謝
編輯。
create table `prova` (
`id` int(11) not null auto_increment,
`id_gruppo` int(11) default null,
`prog` int(11) default null,
primary key (`id`)
) engine=myisam charset=latin1;
delimiter //
drop procedure if exists inserisci //
create procedure inserisci(in quanti int)
begin
declare i int default 0;
while i < quanti do
insert into prova (id_gruppo,prog) values (
(floor(1 + (rand() * 100))),
(floor(1 + (rand() * 30)))
);
set i = i + 1;
end while;
end //
delimiter ;
call inserisci(1000000);
@Clodoaldo: 我的存儲過程
call casualipergruppo('prova','id_gruppo',2);
給了我200條記錄,並需要約23秒。您的存儲過程不斷給我錯誤代碼:1473選擇的嵌套級別太高,即使我將varchar值增加到20000.我不知道查詢中涉及的聯合是否有任何限制。
您可以簡單地從random_prova中的列表中排除選定的行。執行此操作的一種方法是將所選值推入數組中。排除那些數組中的數據。但其他方法也可用。 – 2011-03-09 04:30:07
感謝您的回覆。也許我錯了,但在我看來,你並不認爲我需要每組中有N個不同的記錄。可能會發生ids不連續。我正在尋找一種不涉及任何編程語言的SQL解決方案。 @Syed。我不知道如何有效地實施你的建議。 – 2011-03-10 11:12:01