Top n Window Function queries over a specific subset of data are common in analysis and reporting requirements. Luckily, in MySQL, there are Window functions we can use for this type of query. To be quite honest, you don’t necessarily need Window Functions. You can retrieve those top 3 (or whatever) types of results with a regular SQL query. But, since we have those powerful Window Functions, why not use them? My thoughts exactly! Besides, no one wants a spaghetti code mess of SQL to try and understand. Not to mention, Window functions are often better optimized for querying larger data sets. Continue reading and see example queries for more understanding…
OS and DB used:
- Linux Mint 20 “Ulyana”
- MySQL 8.0.22
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Here is the description of the target table I am working with for this blog post. If you have read any of my other posts, you may recognize this table as I tend to use it widely for example queries. The ‘walking_stats’ table stores my personal walking data, that I track while working on my fitness and to maintain a healthier weight:
| Field | Type | Null | Key | Default | Extra |
| day_walked | date | YES | | NULL | |
| cal_burned | decimal(4,1) | YES | | NULL | |
| miles_walked | decimal(4,2) | YES | | NULL | |
| duration | time | YES | | NULL | |
| mph | decimal(2,1) | YES | | NULL | |
| shoe_id | int | YES | | NULL | |
6 rows in set (0.0034 sec)
There are just over 400 rows of data in the table ‘walking_stats’:
Top n Window Function query: Top 3 days
I store the amount of time – in hours, minutes, and seconds – spent walking in the ‘duration’ column. Suppose I want to find the ‘top 3’ number of rows – meaning the longest walks time-wise – over a 6-month span, broken down by each month? This type of question can be easily answered using Top n Window Function queries.
I’ll make use of a CTE instead of a subquery or derived table and generate the rows based on a
WHERE clause that restricts the results to the year 2020 within the months of January to June.
Let’s look at the overall query:
WITH top_3 AS (
SELECT day_walked, cal_burned, duration,
ROW_NUMBER() OVER(PARTITION BY EXTRACT(MONTH FROM day_walked) ORDER BY duration DESC) AS row_num
WHERE EXTRACT(YEAR FROM day_walked) = 2020 —all walks are in the year 2020
AND EXTRACT(MONTH FROM day_walked) <= 6) —all walks are in the months from January to June
WHERE row_num <= 3;
Below are the query results:
In the screen-shot above, I have segregated the groups of rows using red rectangles to create some distinction between each set of top 3 duration rows for each month.
So how does this query work?
Top n Window Function query: Ranking and partitioning data
To establish a ranking amongst the rows, I use the
ROW_NUMBER() Window function which assigns a continuous, incrementing integer value to rows based on the
ORDER BY clause sort criteria within the
Related: Read, ROW_NUMBER() Window Function – find duplicate values, where I cover examples of using this handy Window Function to remove duplicate rows of data from a table.
For this specific query, I order the rows in descending order (greatest to least) for the ‘duration’ column, placing the lengthier times first in the query results order. The
PARTITION BY clause sub-groups the rows based on each month as returned by the
DATE function call.
Then all that’s left is to filter the returned rows based on the ‘row_num’ column alias in the outer query. I use less than or equal to 3, for a top 3 result, but you could use top n Window Function queries for whatever your specific requirements are at this point.
Window Functions informational resources
Be sure and read any of the below blog posts for more information on Window Functions:
- Rolling sum and average – Window Functions MySQL
- RANK() and DENSE_RANK() differences
- ROWS and RANGE: Windowing clause defaults – learning by example in MySQL.
Window Functions are so powerful and fun to use. What are your favorite Window function tricks? Let me know in the comments below.
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Josh Otwell has a passion to study and grow as a SQL Developer and blogger. Other favorite activities find him with his nose buried in a good book, article, or the Linux command line. Among those, he shares a love of tabletop RPG games, reading fantasy novels, and spending time with his wife and two daughters.
Disclaimer: The examples presented in this post are hypothetical ideas of how to achieve similar types of results. They are not the utmost best solution(s). The majority, if not all, of the examples provided, are performed on a personal development/learning workstation-environment and should not be considered production quality or ready. Your particular goals and needs may vary. Use those practices that best benefit your needs and goals. Opinions are my own.