Top N per group query in Vertica

问题

This is an old problem - looking for the best solution in Vertica. Imagine a table with columns:-

A, B, C, D, E

Columns A-D are ints or varchars and column E is a timestamptz column that has a default value of GETUTCDATE().

Sample content of the table:-

1, 2, "AAA", 4, 1404305559
1, 2, "BBB", 23, 1404305633
1, 2, "CCC", 62, 1404305705  <-- the max entry for (1,2,"CCC")
1, 2, "AAA", 123, 1404305740 <-- the max entry for (1,2,"AAA")
1, 2, "BBB", 91, 1404305778  <-- the max entry for (1,2,"BBB")

So potentially there are repeating rows for the composite (A,B,C) value (with column D being a value and column E the timestamp).

I'd like a resultset that showed, for each unique (A,B,C) combination, the latest row and its value. Hence the resultset for the above would look like:-

1, 2, "CCC", 62, 1404305705  
1, 2, "AAA", 123, 1404305740 
1, 2, "BBB", 91, 1404305778

回答1:

Let's set up the sample data:

CREATE TABLE public.test (
  A int,
  B int,
  C varchar,
  D int, 
  E int
);

INSERT INTO public.test (A, B, C, D, E) VALUES (1, 2, 'AAA', 4, 1404305559);
INSERT INTO public.test (A, B, C, D, E) VALUES (1, 2, 'BBB', 23, 1404305633);
INSERT INTO public.test (A, B, C, D, E) VALUES (1, 2, 'CCC', 62, 1404305705);
INSERT INTO public.test (A, B, C, D, E) VALUES (1, 2, 'AAA', 123, 1404305740);
INSERT INTO public.test (A, B, C, D, E) VALUES (1, 2, 'BBB', 91, 1404305778);

COMMIT;

We'll use the RANK function to rank each row based on A, B, C and sort on E and return only the rows that are at the top (have a rank of 1).

SELECT a.a, 
       a.b, 
       a.c, 
       a.d, 
       a.e 
FROM   (SELECT a, 
               b, 
               c, 
               d, 
               e, 
               RANK() 
                 OVER ( 
                   PARTITION BY a, b, c 
                   ORDER BY e DESC) AS rank 
        FROM   public.test) a 
WHERE  a.rank = 1;

This returns:

 A | B |  C  |  D  |     E
---+---+-----+-----+------------
 1 | 2 | CCC |  62 | 1404305705
 1 | 2 | AAA | 123 | 1404305740
 1 | 2 | BBB |  91 | 1404305778

来源：https://stackoverflow.com/questions/24531759/top-n-per-group-query-in-vertica

标签

vertica