sql查询返回的记录数限制

Question

如果“select * from table;”给出以下结果， 我怎样才能限制它只显示每个ref的前3条记录？

感谢

ref other field 
---------------------- 
1234 a 
1234 b 
1234 c 
1234 d 
1234 e 
5678 a 
5678 b 
5678 c 
5678 d 

Answer 1

对于Microsoft SQL Server 2005+：

SELECT ref, OtherField 
    FROM (SELECT ref, OtherField, 
       ROW_NUMBER() OVER(PARTITION BY ref ORDER BY OtherField) AS RowNum 
       FROM YourTable) t 
    WHERE t.RowNum <= 3 

同样的查询，使用CTE：

WITH cteRowNum AS (
    SELECT ref, OtherField, 
      ROW_NUMBER() OVER(PARTITION BY ref ORDER BY OtherField) AS RowNum 
     FROM YourTable 
) 
SELECT ref, OtherField 
    FROM cteRowNum 
    WHERE RowNum <= 3 

Answer 2

借助Oracle：

WITH subquery as 
(
    SELECT ref, row_number() over (partition by ref) rank, other_field 
    FROM my_table 
) 
SELECT ref, other_field from subquery where rank <= 3; 

查找为Oracle analytic functions。

Answer 3

将在SQL Server的工作另一种解决方案2005+

Select ... 
From MyTable As T 
    Cross Apply (
       Select TOP 3 T2.OtherField 
       From MyTable As T2 
       Where T2.ref = T.ref 
       Order By T2.OtherField 
       ) As Z