Select at least one from each category?

Question

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I\'ve got an SQLite database with a bunch of test/exam questions. Each question belongs to one question category.

Answer 1

The key to the answer is that there are two kinds of questions in the result: for each category, one question that must be constrained to come from that category; and some remaining questions.

First, the constrained questions: we just select one record from each category:

SELECT id, category_id, question_text, 1 AS constrained, max(random()) AS r
FROM so_questions
GROUP BY category_id

(This query relies on a feature introduced in SQLite 3.7.11 (in Jelly Bean or later): in a query SELECT a, max(b), the value of a is guaranteed to come from the record that has the maximum b value.)

We also have to get the non-constrained questions (filtering out the duplicates that are already in the constrained set will happen in the next step):

SELECT id, category_id, question_text, 0 AS constrained, random() AS r
FROM so_questions

When we combine these two queries with UNION and then group by the id, we have all the duplicates together. Selecting max(constrained) then ensures that for the groups that have duplicates, only the constrained question remains (while all the other questions have only one record per group anyway).

Finally, the ORDER BY clause ensures that the constrained questions come first, followed by some random other questions:

SELECT *, max(constrained)
FROM (SELECT id, category_id, question_text, 1 AS constrained, max(random()) AS r
      FROM so_questions
      GROUP BY category_id
      UNION ALL
      SELECT id, category_id, question_text, 0 AS constrained, random() AS r
      FROM so_questions)
GROUP BY id
ORDER BY constrained DESC, r
LIMIT 5

---

For earlier SQLite/Android versions, I haven't found a solution without using a temporary table (because the subquery for the constrained question must be used multiple times, but does not stay constant because of the random()):

BEGIN TRANSACTION;

CREATE TEMPORARY TABLE constrained AS
SELECT (SELECT id
        FROM so_questions
        WHERE category_id = cats.category_id
        ORDER BY random()
        LIMIT 1) AS id
FROM (SELECT DISTINCT category_id
      FROM so_questions) AS cats;

SELECT ids.id, category_id, question_text
FROM (SELECT id
      FROM (SELECT id, 1 AS c
            FROM constrained
            UNION ALL
            SELECT id, 0 AS c
            FROM so_questions
            WHERE id NOT IN (SELECT id FROM constrained))
      ORDER BY c DESC, random()
      LIMIT 5) AS ids
JOIN so_questions ON ids.id = so_questions.id;

DROP TABLE constrained;
COMMIT TRANSACTION;