Using LIMIT within GROUP BY to get N results per group?

Using LIMIT within GROUP BY to get N results per group

If you've ever encountered a situation where you need to retrieve a specific number of results per group in your SQL query, you're not alone. The good news is that there is a solution! 🎉

In the given context, the goal is to obtain the top 5 results for each id, considering only the years between 2000 and 2009. Let's break down the problem and explore possible solutions. 🤔

The Challenge

The original query selects the year, id, and rate columns from table h. It applies some conditions using the WHERE clause and then groups the results by id and year. Finally, it orders the results by id and rate in descending order.

The desired output, however, requires that only the top 5 results per id be returned. This means that for each unique id, we want to see only the rows with the highest rate values. Sounds challenging, right? 😓

The Solution

While there isn't a "built-in" solution using a LIMIT-like modifier within the GROUP BY clause, we can achieve the desired outcome by leveraging subqueries and the ROW_NUMBER() function.

The idea is to use a subquery to assign a row number to each row within each group, based on the desired order, and then filter the results to retain only the rows with row numbers less than or equal to 5.

Here's an example query that implements this approach:

SELECT year, id, rate
FROM (
  SELECT year, id, rate, ROW_NUMBER() OVER (PARTITION BY id ORDER BY rate DESC) AS row_num
  FROM h
  WHERE year BETWEEN 2000 AND 2009
    AND id IN (SELECT rid FROM table2)
) AS subquery
WHERE row_num <= 5
ORDER BY id, rate DESC;

What's Happening?

• The subquery within the FROM clause calculates the row number for each row within each group defined by the id column. The PARTITION BY clause ensures that row numbers restart for each unique id, and the ORDER BY clause specifies the order based on the rate column in descending order.

• We then filter the subquery by selecting only the rows where the row number is less than or equal to 5. This gives us the top 5 results per id.

• Finally, the outer query selects the desired columns (year, id, and rate) from the subquery and sorts the results by id and rate in descending order.

In Practice

Using the given example data, running the above query would yield the expected result:

year   id    rate
2006   p01   8
2003   p01   7.4
2008   p01   6.8
2001   p01   5.9
2007   p01   5.3
2001   p02   12.5
2004   p02   12.4
2002   p02   12.2
2003   p02   10.3
2000   p02   8.7

Conclusion

By using a combination of subqueries and the ROW_NUMBER() function, we've successfully achieved the desired outcome of obtaining the top 5 results per group in our SQL query. 🙌

Next time you encounter a similar challenge, remember the power of subqueries and think creatively to solve your SQL problems efficiently. 💪

If you found this solution helpful or have any questions, please leave a comment below. Let's learn and grow together! 🌟