Select From a table limiting by column of another table - mysql

Question

I have a table named question_dispositions composed like this
Type | chapter | number
-------------------------
A | 1 | 3
B | 1 | 2
B | 4 | 1
The second table named question has columns type, text and chapter.
Type | chapter | text
-----------------------
A | 1 | T1
A | 1 | T2
B | 1 | T3
B | 1 | T4
B | 1 | T5
B | 2 | T6
B | 2 | T7
B | 3 | T8
B | 4 | T9
What I need is to fetch X random questions from table Question where the X is the column number from question_dispositions
So if I need questions of type B the result that I expect is this:
chapter | text
--------------
1 | T3
1 | T4
4 | T9
How can I Select random question limiting by the column number in another table? I tried with this query but returns every row in the table Question as expected because I can not limit with qd.count
SELECT * FROM
question_dispositions AS qd, question AS q
WHERE qd.chapter = q.chapter AND qd.type = q.type AND qd.type = 'B'

Answer 1

ORDER BY RAND() is a typical approach to randomness in MySQL but it is usually criticized because it does not scale well. Here however you appear to be selecting a quite small number of rows as questions but to achieve some randomness of those. So, here is an approach using ORDER BY RAND() which should be acceptable in performance for small result sets.
There is an added requirement to LIMIT by a stored Number, however in the small example supplied that number isn't consistent for type B so I chose to limit by the maximum of that number for the wanted type B. MySQL doesn't allow use of variables or subqueries with the LIMIT clause so instead a generated row number is used in a where clause to achieve that outcome.
Refer to this SQL Fiddle
Data:
CREATE TABLE question_dispositions
(`Type` varchar(1), `chapter` int, `number` int)
;
INSERT INTO question_dispositions
(`Type`, `chapter`, `number`)
VALUES
('A', 1, 3),
('B', 1, 2),
('B', 4, 1)
;
CREATE TABLE question
(`Type` varchar(1), `chapter` int, `text` varchar(2))
;
INSERT INTO question
(`Type`, `chapter`, `text`)
VALUES
('A', 1, 'T1'),
('A', 1, 'T2'),
('B', 1, 'T3'),
('B', 1, 'T4'),
('B', 1, 'T5'),
('B', 2, 'T6'),
('B', 2, 'T7'),
('B', 3, 'T8'),
('B', 4, 'T9')
;
Query:
SELECT *
FROM (
SELECT #rownum := #rownum + 1 AS rn , q.*
FROM question_dispositions AS qd
INNER JOIN question AS q ON qd.chapter = q.chapter AND qd.type = q.type
CROSS JOIN (SELECT #rownum := 0) r
WHERE qd.type = 'B'
ORDER BY RAND()
) d
WHERE rn <= (SELECT MAX(NUMBER) FROM question_dispositions WHERE type = 'B')
Example Results:
| rn | Type | chapter | text |
|----|------|---------|------|
| 1 | B | 1 | T3 |
| 2 | B | 1 | T4 |