mysql random selection in inner join - mysql

Question

Question Mysql Random Row Query on Inner Join is much the same as mine but it was never answered.
I have a master table m and slave s. S contains 1 to many rows for each m. I would like a query that selects every master row joined to exactly one randomly chosen slave.
If the table schemas were:
M
---
id
S
---
id
mid
then, in pseudo code the query would be:
select * from m inner join s on m.id = s.mid where s.id is one randomly chosen from the values that exist
Can this be translated into real SQL?

Answer 1

I think the following query does the required job but using a subquery (not inner join):
SELECT *, (SELECT id FROM S WHERE S.mid = M.id ORDER BY RAND() LIMIT 1) AS S_id
FROM M
Here is a link to test it.
Hope it helps.

Answer 2

This can be solved using Row_Number() concept. We need to randomly assign row number values within a partition of mid in the table s. And, do a Join from the m table to s using mid and row_number = 1. This will pick a single Random row everytime.
In MySQL version below 8, we can use User-defined Variables to emulate Row_Number(). To understand how this works, you may check this answer for the explanation: https://stackoverflow.com/a/53465139/2469308
Note that this technique will be efficient on Large tables than using a Subquery (in the SELECT clause), as it will be doing overall table Sorting only once
View on DB Fiddle
create table m (id int, m_nm varchar(10));
create table s (id int,
mid int references m(mid),
s_nm varchar(10));
insert into m values(1, "a");
insert into m values(2, "b");
insert into m values(3, "c");
insert into s values(1, 1, "aa");
insert into s values(2, 1, "aa");
insert into s values(3, 2, "bb");
insert into s values(4, 2, "bbb");
insert into s values(5, 2, "bbbb");
insert into s values(6, 3, "cc");
insert into s values(7, 3, "ccc");
Query
SELECT
m.*, s_dt.id, s_dt.mid, s_dt.s_nm
FROM
m
JOIN
(
SELECT
#rn := IF(#m = dt.mid, #rn+1, 1) AS row_num,
#m := dt.mid AS mid,
dt.id,
dt.s_nm
FROM
(
SELECT
id, mid, s_nm, RAND() as rand_num
FROM s
ORDER BY mid, rand_num ) AS dt
CROSS JOIN (SELECT #rn:=0, #m:=0) AS user_vars
) AS s_dt
ON s_dt.mid = m.id AND
s_dt.row_num = 1;
Result (Run #1)
| id | m_nm | id | mid | s_nm |
| --- | ---- | --- | --- | ---- |
| 1 | a | 2 | 1 | aa |
| 2 | b | 5 | 2 | bbbb |
| 3 | c | 7 | 3 | ccc |
Result (Run #2)
| id | m_nm | id | mid | s_nm |
| --- | ---- | --- | --- | ---- |
| 1 | a | 1 | 1 | aa |
| 2 | b | 4 | 2 | bbb |
| 3 | c | 6 | 3 | cc |
Result (Run #3)
| id | m_nm | id | mid | s_nm |
| --- | ---- | --- | --- | ---- |
| 1 | a | 1 | 1 | aa |
| 2 | b | 3 | 2 | bb |
| 3 | c | 7 | 3 | ccc |
MySQL 8.0.2+ / MariaDB 10.3+ solution would be simply the following:
SELECT
m.*, s_dt.id, s_dt.mid, s_dt.s_nm
FROM
m
JOIN
(
SELECT
s.*,
ROW_NUMBER() OVER w AS row_num
FROM s
WINDOW w AS (PARTITION BY mid
ORDER BY RAND())
) AS s_dt
ON s_dt.mid = m.id AND
s_dt.row_num = 1
View on DB Fiddle