选择具有少量唯一列的记录，同时选择非空值优先于另一列优先

Question

我想通过获取field1和field2唯一的记录来查询表，同时选择field4具有某些值而非优先级为null的行。

例如在原始表

field1 field2 field3 field4 
    1  B  A  (null) 
    1  B  A  2 
    2  C  A  3 
    2  D  A  (null) 
    3  D  F  3 
    2  C  A  3 

而且是我想从输出查询来获取这样的：

field1 field2 field3 field4 
    1  B  A  2 
    2  C  A  3 
    2  D  A  (null) 
    3  D  F  3 

是否存在SQL表的joinning技术/过滤来实现这一目标的任何有效的方法？感谢

P/S - 为了避免混淆，具有不同FIELD1的目的和FIELD2仅是因为字段3可以具有所选择的行将基于那些行其field4中不优先

空不同的值

eg

field1 field2 field3 field4 
    1  B  A  (null) 
    1  B  C  2  <- this will be the chosen one 

Answer 1

试试这个：

你只需要组由前3场，并采取MAX（）的filed4

select "field1","field2","field3",max("field4") 
from Table1 
group by "field1","field2","field3" 
order by "field1" 

SQL fiddle demo

编辑：

如果你想只有FIELD1和FIELD2为分组，然后尝试的一部分，这一点：

select "field1","field2","field3","field4" 
from(
select "field1","field2","field3","field4", 
     row_number() 
     over(partition by "field1","field2" order by "field4" desc) as rn 
from Table1)A 
where A.rn=1 

SQL Fiddle Demo 2

Answer 2

您可以使用最大，但一定要确保你选择正确的字段3。这将做到这一点：

select field1, 
    field2 , 
    max(field3) keep (dense_rank first order by field4 desc) as field3, 
    max(field4) 
from Table1 
group by field1, field2 ,field3, 
order by field1, field2