Question

我有一个问题表，我想从每个类别25个问题中随机选择。

“问题”表格如下：

类别为：C PC P PP

id问题类别answer1 answer2 answer3

Answer 1

最简单的方法是将union all与每个类别的子查询一起使用：

(select * from questions where category = 'C' order by rand() limit 25)
union all
(select * from questions where category = 'PC' order by rand() limit 25)
union all
(select * from questions where category = 'P' order by rand() limit 25)
union all
(select * from questions where category = 'PP' order by rand() limit 25)

如果您有很多类别或大量问题（数十万或更多），那么您可能需要一个性能更好的查询。但是对于较少量的数据，这可能很好。

我想强调union all对于这样的查询比union更好。 union删除重复项，添加在这种情况下不需要的添加处理。

Answer 2

不确定i want to select random from each category你的意思，但你可以从每个类别中获取25个问题并将它们联合起来像

select question from sometable where category = 'C' limit 25
union
select question from sometable where category = 'PC' limit 25
union
select question from sometable where category = 'P' limit 25
union
select question from sometable where category = 'PP' limit 25

Answer 3

使用带有ORDER BY RAND()的外部查询来选择以随机方式从内部查询中选择的记录。

SELECT * FROM
(
    (SELECT * FROM questions WHERE category='C' ORDER BY RAND() LIMIT 25) T1
    UNION
    (SELECT * FROM questions WHERE category='PC' ORDER BY RAND() LIMIT 25) T2
    UNION
    (SELECT * FROM questions WHERE category='P' ORDER BY RAND() LIMIT 25) T3
    UNION
    (SELECT * FROM questions WHERE category='PP' ORDER BY RAND() LIMIT 25) T4
) TT
ORDER BY RAND()

Answer 4

这是您可以使用的HINT。

<强>语法： 使用此查询：

SELECT * FROM `table` WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` )

代替'table' put 'questions'。（这是因为你有一个表名'问题'）。

希望这会有所帮助。

如何从表中随机选择每个类别的25个问题，我有4个类别？

4 个答案: