SQL：從同一表和同一列中選擇，只是計數不同

Question

我有一個名為names的表，我想在被count（*）作為uniq之后選擇2個名字，然后僅從整個樣本池中選擇另外2個名字。

firstname
John
John
Jessica
Mary
Jessica
John
David
Walter

因此，前兩個名稱將從John ， Jessica和Mary等的組合中選擇，從而給他們相等的機會，而后兩個名稱將從整個組合中選擇，因此， John和Jessica明顯偏見將具有多個行。

我敢肯定有辦法做到這一點，但我只是想不通。 我想做類似的事情

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

這可能嗎？ 感謝任何指針。

Answer 1

我認為您很親密，但是您需要一些隨機性進行采樣：

(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

如此處所述，您可以根據數據庫使用RAND或類似函數來實現它。

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)

遵循類似的方法從整個池中進行選擇