Select 5 of each distinct value

问题

I have the following table in PostgreSQL:

|  a  |  b  |  c  |
===================
| 'w' |  2  |  3  |
| 'w' |  7  |  2  |
| 'w' |  8  |  1  |
| 'w' |  3  |  6  |
| 'w' |  0  |  8  |
| 'w' |  2  |  9  |
| 'w' |  2  |  9  |
| 'z' |  4  |  9  |
| 'z' |  0  |  9  |
| 'z' |  0  |  8  |
| 'z' |  3  |  6  |
| 'z' |  2  |  7  |
| 'z' |  3  |  1  |
| 'z' |  3  |  2  |
| 'z' |  3  |  3  |

I want to select all records, but limit them to 5 records for each distinct value in column a.
So the result would look like:

|  a  |  b  |  c  |
===================
| 'w' |  2  |  3  |
| 'w' |  7  |  2  |
| 'w' |  8  |  1  |
| 'w' |  3  |  6  |
| 'w' |  0  |  8  |
| 'z' |  4  |  9  |
| 'z' |  0  |  9  |
| 'z' |  0  |  8  |
| 'z' |  3  |  6  |
| 'z' |  2  |  7  |

What is the most effecient way to achieve that in RoR? Thanks!

回答1:

you can use row_number, but you have to specify order or you will get unpredictable resutls

   with cte as (
       select
           *,
           row_number() over(partition by a order by b, c) as row_num
       from table1
   )
   select a, b, c
   from cte
   where row_num <= 5

来源：https://stackoverflow.com/questions/18231744/select-5-of-each-distinct-value