Mysql select min in userid userid =？

Question

此表存儲誰購買了哪本書以及何時購買的數據。我想知道哪些書是由USERID＃1首先購買的。 SQL fiddle

這將導致

USERID BOOKID DATE_PURCHASE 
1  2  2014-03-22 (purchased only by USERID #1) 
1  5  2014-03-29 (purchased earlier than USERID #2) 
1  6  2014-03-28 (purchased earlier than USERID #3) 
1  7  2014-03-26 (purchased earlier than USERID #3) 

它不顯示BOOKID＃3，因爲這本書是由USERID＃2第一次購買。

請注意，該表可能包含數以萬計的行，所以我需要一個有效的解決方案。

如果不得不做手工，我會做這樣的：

1. Check what books did USERID #1 buy 
2. Go through all rows with those books (BOOKID) and check the MIN(DATE_PURCHASE) for each row 
3. Write out those rows only where the USERID = #1 

Answer 1

要篩選一group-wise minimum，這本身就是使用自聯接（爲了尋找第一次購買之日起制定只有那些期望的用戶買的，不管購買的書籍）：

SELECT * FROM docs NATURAL JOIN (
    SELECT BOOKID, MIN(d2.DATE_PURCHASE) DATE_PURCHASE 
    FROM  docs d1 JOIN docs d2 USING (BOOKID) 
    WHERE d1.USERID = 1 
    GROUP BY BOOKID 
) t WHERE USERID = 1 

看到它的sqlfiddle。