联合返回具有限制 postgresql 的不同输出 [英] Union returning different outputs with limit postgresql

问题描述

我必须根据难度级别从我的问题集中提取 12 个问题.以下是我写的查询.

I have to fetch 12 questions from my question set based on the difficulty level. Following is the query I have written.

(SELECT q.question_text, q.option_a, q.option_b, q.option_c, q.option_d,
        q.correct_answer, q.image_link, q.question_type
        FROM questions_bank q
        JOIN sports_type st ON st.id = q.sports_type_id
        JOIN difficulty_level dl ON dl.id = q.difficulty_level_id
        WHERE st.game_type = LOWER('cricket') AND dl.value = 'E'
        ORDER BY random()
        LIMIT 7) 
UNION 
(SELECT q.question_text, q.option_a, q.option_b, q.option_c, q.option_d,
        q.correct_answer, q.image_link, q.question_type
        FROM questions_bank q
        JOIN sports_type st ON st.id = q.sports_type_id
        JOIN difficulty_level dl ON dl.id = q.difficulty_level_id
        WHERE st.game_type = LOWER('cricket') AND dl.value = 'M'
        ORDER BY random()
        LIMIT 4)
UNION 
(SELECT q.question_text, q.option_a, q.option_b, q.option_c, q.option_d, 
        q.correct_answer, q.image_link, q.question_type
        FROM questions_bank q
        JOIN sports_type st ON st.id = q.sports_type_id
        JOIN difficulty_level dl ON dl.id = q.difficulty_level_id
        WHERE st.game_type = LOWER('cricket') AND dl.value = 'H'
        ORDER BY random()
        LIMIT 1);

问题是，每当我运行这个查询时，每次它都会给我不同数量的结果，而不是静态的 12.有时我得到 12，有时 10，有时 15.我希望输出中有 12 行，而不是更少，不多.

The issue is that whenever I run this query, each time it gives me a different number of results instead of the static 12. Sometimes I get 12, sometimes 10, sometimes 15. I expect 12 rows in output, not less, not more.

这个查询有什么问题?

推荐答案

使用窗口函数可以简化为单个选择:

It can be reduced to a single select by using a window function:

select *
from (
    select
        row_number() over (partition by dl.value order by random()) as rn,
        dl.value,
        q.question_text, q.option_a, q.option_b, q.option_c, q.option_d,
        q.correct_answer, q.image_link, q.question_type
    from
        questions_bank q
        inner join
        sports_type st on st.id = q.sports_type_id
        inner join
        difficulty_level dl on dl.id = q.difficulty_level_id
    where st.game_type = lower('cricket') and dl.value in ('E','M','H')
) s
where 
    value = 'E' and rn <= 7 or
    value = 'M' and rn <= 4 or
    value = 'H' and rn = 1

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