Get top n records for each group of grouped results

Gearing up for tied scores

Ties do happen! Here's how you adjust the approach to include all records tied up to the Nth position:

WITH ranked AS (
  SELECT *, DENSE_RANK() OVER (PARTITION BY group_col ORDER BY rank_col DESC) as dense_rank
  FROM table_name
)
SELECT * FROM ranked WHERE dense_rank <= N;


DENSE_RANK(): Because nobody likes to be left out when scores are the same. 😎

Scaling up for larger datasets

When you're dealing with massive data, efficiency is king. A slower function might become a drag as the data grows. Here's an optimized query designed for speed:

SELECT t1.*
FROM table_name t1
LEFT JOIN table_name t2
  ON t1.group_col = t2.group_col 
  AND t1.rank_col < t2.rank_col
GROUP BY t1.id
HAVING COUNT(DISTINCT t2.id) < N;


This query is the Usain Bolt of SQL: Fast and efficient on larger datasets. 🏃‍♂️💨

ROW_NUMBER() magic for older SQL versions

There was a time before MySQL 8.0 and when databases lacked window functions. Fear not, here is how to emulate ROW_NUMBER with session variables in those times:

SELECT *
FROM (
  SELECT 
    @row_num := IF(@prev_value=group_col, @row_num + 1, 1) AS row_num,
    @prev_value := group_col,
    t.*
  FROM 
    (SELECT * FROM table_name ORDER BY group_col, rank_col DESC) t
  JOIN 
    (SELECT @row_num := 0, @prev_value := NULL) r
) ranked
WHERE row_num <= N;


It's like a homemade version of ROW_NUMBER(). A true SQL chef! 👨‍🍳

Preparing for large groups and bypassing pitfalls

Handling large groups could cause performance issues. Techniques like indexing your group_col and rank_col can greatly boost query performance. Rename identifiers to avoid using reserved keywords.

Even SQL can sometimes forget the order of things (poor MySQL 5.6). Here's a nested select fix to ensure your ORDER BY isn't forgotten:

SELECT * FROM (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY group_col ORDER BY rank_col DESC) as row_num
  FROM (
    SELECT * FROM table_name ORDER BY group_col, rank_col DESC
  ) tmp_ordered
) tmp WHERE row_num <= N;


Exploring common alternatives and their benefits

Different databases, different rules. For example, PostgreSQL presents the DISTINCT ON expression for unique records based on order.

Also, consider Common Table Expressions (CTEs) or temporary tables for complex queries. Think of it as keeping your codebase tidy!

Display comparison results clearly

Output clarity is crucial. Use AS to give alternative names for columns. Showing data in columnar mode as SQL clients do ensures detailed comparison results.