I have a table called names
, and I want to select 2 names after being count(*) as uniq, and then another 2 names just from the entire sample pool.
firstname
John
John
Jessica
Mary
Jessica
John
David
Walter
So the first 2 names would select from a pool of John
, Jessica
, and Mary
etc giving them equal chances of being selected, while the second 2 names will select from the entire pool, so obvious bias will be given to John
and Jessica
with multiple rows.
I'm sure there's a way to do this but I just can't figure it out. I want to do something like
SELECT uniq.firstname
FROM (SELECT firstname, count(*) as count from names GROUP BY firstname) uniq
limit 2
AND
SELECT firstname
FROM (SELECT firstname from names) limit 2
Is this possible? Appreciate any pointers.
I think you are close but you need some randomness for the sampling:
(SELECT uniq.firstname
FROM (SELECT firstname, count(*) as count from names GROUP BY firstname) uniq
ORDER BY rand()
limit 2
)
UNION ALL
(SELECT firstname
FROM from names
ORDER BY rand()
limit 2
)
As mentioned here you can use RAND or similar functions to achieve it depending on the database.
MySQL:
SELECT firstname
FROM (SELECT firstname, COUNT(*) as count FROM names GROUP BY firstname)
ORDER BY RAND()
LIMIT 2
PostgreSQL:
SELECT firstname
FROM (SELECT firstname, COUNT(*) as count FROM names GROUP BY firstname)
ORDER BY RANDOM()
LIMIT 2
Microsoft SQL Server:
SELECT TOP 2 firstname
FROM (SELECT firstname, COUNT(*) as count FROM names GROUP BY firstname)
ORDER BY NEWID()
IBM DB2:
SELECT firstname , RAND() as IDX
FROM (SELECT firstname, COUNT(*) as count FROM names GROUP BY firstname)
ORDER BY IDX FETCH FIRST 2 ROWS ONLY
Oracle:
SELECT firstname
FROM(SELECT firstname, COUNT(*) as count FROM names GROUP BY firstname ORDER BY dbms_random.value )
WHERE rownum in (1,2)
Follow the similar approach for selecting from entire pool
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