Question

我有一个名为names的表，我想在count（*）之后选择2个名称作为uniq，然后再从整个样本池中选择另外2个名称。

firstname
John
John
Jessica
Mary
Jessica
John
David
Walter

因此，前两个名称将从John，Jessica和Mary等池中进行选择，从而为他们提供相同的选择机会，而后两个名称将从整个池，因此John和Jessica有多行显而易见的偏见。

我确信有办法做到这一点，但我无法理解。我想做像

这样的事情

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

我认为你很接近，但你需要一些随机性来进行抽样：

(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

如上所述here，您可以使用RAND或类似功能来实现它，具体取决于数据库。

MySQL的：

SELECT firstname 
FROM (SELECT firstname, COUNT(*) as count FROM names GROUP BY firstname) 
ORDER BY RAND()
LIMIT 2

的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)

按照类似的方法从整个池中进行选择

SQL：从同一个表和同一列中选择，只是不同的计数

2 个答案: