SQL：从同一表和同一列中选择，只是计数不同

Question

我有一个名为names的表，我想在被count（*）作为uniq之后选择2个名字，然后仅从整个样本池中选择另外2个名字。

firstname
John
John
Jessica
Mary
Jessica
John
David
Walter

因此，前两个名称将从John ， Jessica和Mary等的组合中选择，从而给他们相等的机会，而后两个名称将从整个组合中选择，因此， John和Jessica明显偏见将具有多个行。

我敢肯定有办法做到这一点，但我只是想不通。 我想做类似的事情

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

这可能吗？ 感谢任何指针。

Answer 1

我认为您很亲密，但是您需要一些随机性进行采样：

(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

如此处所述，您可以根据数据库使用RAND或类似函数来实现它。

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

甲骨文：

SELECT firstname 
FROM(SELECT firstname, COUNT(*) as count FROM names GROUP BY firstname ORDER BY dbms_random.value )
WHERE rownum in (1,2)

遵循类似的方法从整个池中进行选择