How to Retrieve the Top N Oldest People from Each Group in a Dataset?

Efficiently Selecting the Oldest Individuals from Each Group

This article demonstrates two methods for retrieving the top N (in this case, two) oldest individuals from each group within a dataset. The first utilizes a UNION ALL approach, while the second employs row numbering for a more scalable solution.

Method 1: UNION ALL (Suitable for a small, fixed number of groups)

This method is straightforward for a known and limited number of groups. It selects the top N oldest individuals from each group individually and combines the results. However, this approach becomes cumbersome and inefficient as the number of groups increases.

(
  SELECT *
  FROM mytable
  WHERE `group` = 1  -- Group 1
  ORDER BY age DESC
  LIMIT 2
)
UNION ALL
(
  SELECT *
  FROM mytable
  WHERE `group` = 2  -- Group 2
  ORDER BY age DESC
  LIMIT 2
)
-- ... Add more UNION ALL clauses for additional groups

Method 2: Row Numbering (Scalable solution for a large or unknown number of groups)

This method dynamically assigns a row number to each individual within their respective groups, ordered by age in descending order. This allows for efficient selection of the top N oldest individuals across all groups.

SELECT person, `group`, age
FROM
(
   SELECT person, `group`, age,
      (@num:=IF(@group = `group`, @num +1, IF(@group := `group`, 1, 1))) AS row_number
  FROM test t
  CROSS JOIN (SELECT @num:=0, @group:=NULL) c
  ORDER BY `group`, Age DESC, person
) AS x
WHERE x.row_number <= 2 -- Select the top 2 from each group

This approach leverages variables @num and @group to track the row number and group, respectively. The CROSS JOIN initializes these variables. The final WHERE clause filters the results to only include the top 2 individuals from each group. This method is significantly more efficient and adaptable than the UNION ALL approach for datasets with many groups.

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