需要知道，如果这是可能的SQL如果是的话，该怎么办呢

Question

这里是我当前的查询：

SELECT questions.question, questions_headings.title FROM questions JOIN questions_headings ON questions.heading=questions_headings.id WHERE questions.heading IN (1,2,3) ORDER BY RANDOM() LIMIT 10 

基本上，该数据库包含各种标题问题。例如：

questions_headings: 

+----+-------+ 
| id | title | 
+----+-------+ 
| 0 | blah1 | 
+----+-------+ 
| 1 | lol1 | 
+----+-------+ 
| 2 | etc1 | 
+----+-------+ 

questions: 

+----+---------+----------+ 
| id | heading | question | 
+----+---------+----------+ 
| 0 | 1  | howdoi | 
+----+---------+----------+ 
| 1 | 0  | blahques | 
+----+---------+----------+ 
| 2 | 1  | herro | 
+----+---------+----------+ 

我的查询所做的是从给定标题中随机选择X个问题并将其显示给用户。

目前，如果你想要10个随机问题（限制10），它会给你10个随机问题跨所有ID。正常，对吧？但我不想要这个。

我需要查询的是，抽出10个随机问题分布在给定的ID。那样的话，我不会以一个标题的9个问题和另一个问题的1个问题结束。

希望这是有道理的......

是否有可能只用SQL做到这一点？

Answer 1

如果在创建查询时知道的标题，你可以这样做以下：

SELECT * FROM (
    SELECT questions.question, questions_headings.title FROM questions JOIN questions_headings ON questions.heading=questions_headings.id WHERE questions.heading = 1 ORDER BY RANDOM() LIMIT 3 
    UNION 
    SELECT questions.question, questions_headings.title FROM questions JOIN questions_headings ON questions.heading=questions_headings.id WHERE questions.heading = 2 ORDER BY RANDOM() LIMIT 3 
    UNION 
    SELECT questions.question, questions_headings.title FROM questions JOIN questions_headings ON questions.heading=questions_headings.id WHERE questions.heading = 3 ORDER BY RANDOM() LIMIT 3 
    UNION 
    SELECT questions.question, questions_headings.title FROM questions JOIN questions_headings ON questions.heading=questions_headings.id WHERE questions.heading IN (1,2,3) ORDER BY RANDOM() LIMIT 10 
) LIMIT 10 

的想法是，让10/3 = 3，从每一个标题，然后得到的残留物（由于舍入错误）。因为最后一次查询可能导致已经获取的数据，所以我们限制为10，所以我们明确地获得我们需要的额外1，然后再次限制整个事情。这应该导致均匀分布的值。

Answer 2

下面将在DB2工作（并可以很容易地移植到其他数据库支持ROW_NUMBER）：

SELECT * FROM (
    SELECT question, title 
    FROM (
    SELECT questions.question, questions_headings.title, 
     row_number() over(PARTITION BY questions.headings ORDER BY rand()) rn 
    FROM questions JOIN questions_headings ON questions.heading=questions_headings.id 
    WHERE questions.heading IN (1,2,3) 
) 
    WHERE rn <= 3 
    UNION 
    SELECT questions.question, questions_headings.title 
    FROM questions JOIN questions_headings ON questions.heading=questions_headings.id 
    WHERE questions.heading = 1 
    ORDER BY RAND() 
    FETCH FIRST 10 ROWS ONLY 
) 
FETCH FIRST 10 ROWS ONLY 

Answer 3

这适用于PostgreSQL的：

SELECT q2.row - (SELECT count(*) 
       FROM questions q3 
       WHERE q3.heading IN (1, 2, 3) 
       AND q3.heading < q2.heading) AS ord, 
     q2.heading, q2.question 
FROM (
    SELECT row_number() OVER (ORDER BY heading, random()) as row, 
     q1.id, q1.heading, q1.question 
    FROM questions q1 
    WHERE heading IN (1,2,3) 
    ORDER BY heading, row) AS q2 
ORDER BY ord 
LIMIT 10; 

我无法测试的MySQL版本，但这应该可能是：

SELECT q2.row - (SELECT count(*) 
       FROM questions q3 
       WHERE q3.heading IN (1, 2, 3) 
       AND q3.heading < q2.heading) AS ord, 
     q2.heading, q2.question 
FROM (
    SELECT @rownum:=@rownum+1 AS row, 
     q1.id, q1.heading, q1.question 
    FROM questions q1, (SELECT @rownum:=0) r 
    WHERE heading IN (1,2,3) 
    ORDER BY heading, rand()) AS q2 
ORDER BY ord 
LIMIT 10; 

问题的关键是要记录，以便有ex来自任何选定标题的第二个问题之前的每个选定标题中的一个问题，等等。如果对某些标题没有足够的问题，它仍然会在其他标题上平均分配。