Top-N by group

Top-N queries return only the N top-most or the N bottom-most records from a table or view based on a condition.

In RisingWave, a Top-N query includes a ranking function clause and a rank filtering condition. In the ranking function clause, you can include a PARTITION BY clause to fetch top N rows per group.

Syntax

SELECT [column_list]
  FROM (
    SELECT [column_list],
      ranking_function_clause AS rank
    FROM table_name)
WHERE rank_range;

The syntax of the ranking_function_clause is:

function_name() OVER ([PARTITION BY col1[, col2...]]
        ORDER BY col1 [ ASC | DESC ][, col2 [ ASC | DESC ]...])

rank cannot be included in column_list.

You must follow the pattern exactly to construct a valid Top-N query.

Parameter	Description
function_name	RisingWave supports two window functions in top-N queries: row_number(): Returns the sequential row ordinal (1-based) of each row for each ordered partition.rank(): Returns the ordinal (1-based) rank of each row within the ordered partition. All peer rows receive the same rank value. The next row or set of peer rows receives a rank value which increments by the number of peers with the previous rank value.
PARTITION BY clause	Specifies the partition columns. Each partition will have a Top-N result.
ORDER BY clause	Specifies how the rows are ordered.
rank_range	Specifies the range of the rank number. The rank range is required for the query to be recognized as a top-N query. The range can be specified in these forms. Examples: WHERE M < rank AND rank < N or WHERE rank between M and N. Optionally, you can specify any additional conditions to further filter the results.

Example

Create a table

CREATE TABLE t (x int, y int, z int);

Insert data

INSERT INTO t (x, y, z) VALUES
  (1, 10, 50),
  (1, 10, 60),
  (1, 10, 70),
  (1, 11, 55),
  (1, 11, 65),
  (2, 20, 30),
  (2, 20, 40),
  (2, 21, 25),
  (2, 21, 35),
  (2, 21, 45);

Run a top-N query

SELECT r
  FROM (
    SELECT
      *,
      row_number() OVER (PARTITION BY x ORDER BY y) r
    FROM t
  ) Q
WHERE Q.r < 10;