如何選擇一組唯一的隨機記錄？

Question

今天，我一直在努力滿足這一SQL查詢要求，我想知道是否有人可以幫助我。

我有一個體育問題表。 其中一列是與該問題相關的團隊。 我的要求是返回一定數量的隨機問題，這些問題在團隊中是獨一無二的。

因此，假設我們有下表，並要回答5個問題：

Question        Answer        Team
-----------------------------------
question 1      answer 1      team A
question 2      answer 2      team B
question 3      answer 3      team B
question 4      answer 3      team D
question 5      answer 3      team A
question 6      answer 3      team C
question 7      answer 3      team F
question 8      answer 3      team C
question 9      answer 3      team G
question 10     answer 3      team D

有效結果將返回：

question 1      answer 1      team A
question 2      answer 2      team B
question 4      answer 3      team D
question 6      answer 3      team C
question 7      answer 3      team F

我認為應該可以通過巧妙地使用Distinct和Take來作為干凈的SQL語句來完成此操作，但是我還無法正確地做到這一點。

到目前為止，最好的解決方案是Mladen Prajdic 。 我剛剛對其進行了少許更新以改善其隨機性：

SELECT TOP 10 * 
FROM    (SELECT ROW_NUMBER() OVER(PARTITION BY Team ORDER BY Team, NEWID()) AS RN, *
    FROM Question
    ) teams
WHERE   RN = 2
ORDER BY NEWID()

Answer 1

對於sql 2005，您可以執行以下操作：

select top 5 * 
from    (
            select ROW_NUMBER() over(partition by team order by team) as RN, *
            from @t 
        ) t
where RN = 1
order by NEWID()

Answer 2

這應該在oracle中完成您需要的操作； 對於其他數據庫，顯然，您將需要使用其隨機數源。 可能有更好的方法。 希望其他人會向我們指出：p

select question, answer, team
from
(
select question, answer, team, r
from
(
select 
    question, 
    answer, 
    team,
    rank() over (partition by team order by dbms_random.value) r 
from questions
)
where r = 1
order by dbms_random.value
) where rownum<=5;

測試代碼：

create table questions(question varchar2(16), answer varchar2(16), team varchar2(16));

insert into questions(question, answer, team)
values ('question 1',      'answer 1',      'team A');

insert into questions(question, answer, team)
values ('question 2',      'answer 2',      'team B');

insert into questions(question, answer, team)
values ('question 3',      'answer 3',      'team B');

insert into questions(question, answer, team)
values ('question 4',      'answer 3',      'team D');

insert into questions(question, answer, team)
values ('question 5',      'answer 3',      'team A');

insert into questions(question, answer, team)
values ('question 6',      'answer 3',      'team C');

insert into questions(question, answer, team)
values ('question 7',      'answer 3',      'team F');

insert into questions(question, answer, team)
values ('question 8',      'answer 3',      'team C');

insert into questions(question, answer, team)
values ('question 9',      'answer 3',      'team G');

insert into questions(question, answer, team)
values ('question 10',    'answer 3',      'team D');

commit;

Answer 3

在PostgreSQL（具有獨特的功能）中，我可能會做這樣的事情：

select distinct on (Team) Question, Answer, Team from test order by Team, random() limit 5;

剛剛測試過。 似乎可以工作。