重複刪除

Question

可能重複：
Delete duplicate records from a SQL table without a primary key

我有數據：

SELECT 
      a 
     , b 
    FROM 
    (
     select a = 1, b = 30 
     union all 
     select a = 2, b = 50 
     union all 
     select a = 3, b = 50 
     union all 
     select a = 4, b = 50 
     union all 
     select a = 5, b = 60 
    ) t 

我有一個）dublicate記錄，以獲取輸出（下一個（順序應從結果集中排除）：

a   b 
----------- ----------- 
1   30 
2   50 
3   50 -- should be excluded 
4   50 -- should be excluded 
5   60 

Answer 1

SELECT 
      min(a) as a 
     , b 
    FROM 
    (
     select a = 1, b = 30 
     union all 
     select a = 2, b = 50 
     union all 
     select a = 3, b = 50 
     union all 
     select a = 4, b = 50 
     union all 
     select a = 5, b = 60 
    ) t 
GROUP BY b  
ORDER BY a 

Answer 2

在oracle中我能夠做到這一點使用GROUP BY子句，你應該能夠做到類似。

select min(a), b 
from (select 1 a, 30 b 
     from dual 
     union all 
     select 2 a, 50 b 
     from dual 
     union all 
     select 3 a, 50 b 
     from dual 
     union all 
     select 4 a, 50 b 
     from dual 
     union all 
     select 5 a, 60 b from dual) 
group by b; 

編輯：看起來像其他人想出了一個MS SQL的解決方案，我會在這裏雖然離開這個給後人。

Answer 3

做到這一點，最簡單的方法是用一個簡單的GROUP BY：

SELECT 
     a 
    , b 
INTO #tmp 
FROM 

(
    select a = 1, b = 30 
    union all 
    select a = 2, b = 50 
    union all 
    select a = 3, b = 50 
    union all 
    select a = 4, b = 50 
    union all 
    select a = 5, b = 60 
) t 


SELECT DISTINCT MIN(a) AS a,b 
FROM #tmp 
GROUP BY b 
ORDER BY a