Oracle 随机段 select

Question

我想select下面的片段。

Random 5500 rows including the following segments:
       Subcategorie (sex): - 3300 men
                           - 2200 women
              Subcategorie (age): - 2140 between 18-34 years
                                  - 2100 between 35-54 years
                                  - 1260 between 55-99 years

如何在 select 语句中解决这个问题？

Answer 1

问题是，您使用 "random" 这个词，但您按年龄和性别对群组进行了非常精确的细分。真正随机的单个查询不会产生如此精确的配额。所以你的查询一定很复杂：你需要将整个 table 分成满足你的约束的子集，然后从这些子集中随机 select 。像这样...

select * from (
   select * from whatever
   where sex = 'M'
   and age between 18 and 34
   order by dbms_random.value 
)
where rownum <= 1284
union all
select * from (
   select * from whatever
   where sex = 'M'
   and age between 35 and 54
   order by dbms_random.value 
)
where rownum <= 1260
union all    select * from (
   select * from whatever
   where sex = 'M'
   and age between 55 and 99
   order by dbms_random.value 
)
where rownum <= 756
union all
select * from (
   select * from whatever
   where sex = 'F'
   and age between 18 and 34
   order by dbms_random.value 
)
where rownum <= 856
union all
select * from (
   select * from whatever
   where sex = 'F'
   and age between 35 and 54
   order by dbms_random.value 
)
where rownum <= 840
union all    select * from (
   select * from whatever
   where sex = 'F'
   and age between 55 and 99
   order by dbms_random.value 
)
where rownum <= 504

这可能会表现不佳，具体取决于常见因素 - table 的大小、索引等 - 但它会产生那些确切的同类群组。

如果不是很明显，rownum 界限是每个年龄组的命中数乘以男女比例 (3:2)。

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