SQL：从同一表和同一列中选择，只是计数不同

Question

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.

Answer 1

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
)

Answer 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