How to enforce a max limit no. of rows per day per date in SQL?

Question

Given a data that looks as follows where the date is in string format YYYYMMDD :

item	vietnamese	cost	unique_id	sales_date
fruits	trai cay	10	abc123	20211001
fruits	trai cay	8	foo99	20211001
fruits	trai cay	9	foo99	20211001
vege	rau	3	rr1239	20211001
vege	rau	3	rr1239	20211001
fruits	trai cay	12	abc123	20211002
fruits	trai cay	14	abc123	20211002
fruits	trai cay	8	abc123	20211002
fruits	trai cay	5	foo99	20211002
vege	rau	8	rr1239	20211002
vege	rau	1	rr1239	20211002
vege	rau	12	ud9213	20211002
vege	rau	19	r11759	20211002
fruits	trai cay	6	foo99	20211003
fruits	trai cay	2	abc123	20211003
fruits	trai cay	12	abc123	20211003
vege	rau	1	ud97863	20211003
vege	rau	9	r112359	20211003
fruits	trai cay	6	foo99	20211004
fruits	trai cay	2	abc123	20211004
fruits	trai cay	12	abc123	20211004
vege	rau	9	r112359	20211004

The goal is sample all the rows within a certain time frame, eg 2020-10-02 to 2020-10-03 and to extract a maximum of 3 rows per day, eg with this query:

SELECT * FROM mytable
WHERE sales_date BETWEEN '20211002' AND '20211003'
ORDER BY RAND () LIMIT 6

the expected output for the table above is:

item	vietnamese	cost	unique_id	sales_date
fruits	trai cay	8	abc123	20211002
fruits	trai cay	5	foo99	20211002
vege	rau	8	rr1239	20211002
fruits	trai cay	12	abc123	20211003
vege	rau	1	ud97863	20211003
vege	rau	9	r112359	20211003

But there is a possibility that all 6 rows expected comes from a single day:

item	vietnamese	cost	unique_id	sales_date
fruits	trai cay	12	abc123	20211002
fruits	trai cay	14	abc123	20211002
fruits	trai cay	8	abc123	20211002
fruits	trai cay	5	foo99	20211002
vege	rau	8	rr1239	20211002
vege	rau	1	rr1239	20211002

So to ensure that I have max 3 rows a day, I'm running multiple queries per day, ie

SELECT * FROM mytable
WHERE sales_date='20211002'
ORDER BY RAND () LIMIT 3

and

SELECT * FROM mytable
WHERE sales_date='20211003'
ORDER BY RAND () LIMIT 3

Is there a way to ensure N no. of max limit rows per day in a single query?

Otherwise is there a way to combine those one query per day into a "super-query"? If we're talking about a full year, it'll 365 queries, one per day.

Answer 1

Since 6 rows over 2 days means exactly 3 rows per day, let's expand it to a week.

In a subquery use row_number to assign a number to each row for each date. Then only select those with a row number of 3 or less.

select *
from (
  select
    *,
    row_number() over (partition by sales_date order by rand()) as row
  from mytable
  where sales_date between '20211002' and '20211009'
)
where row <= 3
order by rand()
limit 6