MySQL：如何找出哪一行具有更大的值？

Question

最終，我試圖刪除重複的行，並保留與更大的數字數據的行。因此，要找到重複的數據，我會用：

SELECT * FROM tbl GROUP BY a HAVING COUNT(a)>1

現在，我如何才能找到在field_a列中的大量的行？

Answer 1

也許（排序一種逃避的）：

DELETE t 
FROM t 
EXCEPT (max-query-on-t) 

而且考慮當記錄具有相同的最大值。

Answer 2

delete tbl from tbl, 
(
    select a, max(field_a) as max_a 
    from tbl 
    group by a 
    having count(a)>1 
) as max_for_a 
where tbl.a=max_for_a.a and tbl.field_a!=max_for_a.max_a; 

，如果你有field_a爲每一個兩個或更多更大的價值，就無法刪除所有

來解決這個問題，你需要運行像

select a, count(*) from tbl group by a; 

另一個查詢

，併爲每一個

delete from tbl where a=? limit (total_count-1); 

Answer 3

要做到這一切在一個查詢中，你需要一個唯一的密鑰，你必須存儲臨時表中的中間結果：

delete from tbl where tbl.id in (select result.id from (select tbl.id from tbl join (select max(a) as a from tbl) as maxa where tbl.a<>maxa.a) as result) 

雖然，我會建議將它分成兩個或三個單獨的查詢。