MySQL在與其它的最大列一列不同

Question

比方說，我有這個疑問：

SELECT report_id, time_sent 
FROM report_submissions 
WHERE report_id IN (1,2,3,4,5,6) 
ORDER BY report_id ASC, time_sent DESC 

這個結果：

report_id time_sent 
1   2 
1   1 
2   4 
2   3 
3   4 

而且我想改變這種狀況的查詢，所以我會得到一個DISTINCT report_id及其最大值（time_sent），例如：

report_id time_sent 
1   2 
2   4 
3   4 

如何以最有效的方式執行此操作？謝謝。

Answer 1

SELECT report_id, max(time_sent) 
FROM report_submissions 
WHERE report_id IN (1,2,3,4,5,6) 
GROUP BY report_id 
ORDER BY report_id ASC 

Answer 2

DISTINCT是一個懶惰的GROUP BY！

SELECT report_id, max(time_sent) 
FROM report_submissions 
WHERE report_id IN (1,2,3,4,5,6) 
GROUP BY report_id 
ORDER BY report_id ASC, max(time_sent) DESC 

Answer 3

你需要使用group by

見：http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html