How Can I Retrieve the Top N Rows for Each Group in SQL Using Window Functions?

Use window function to get the first N rows of each group

This article describes how to use SQL window functions to efficiently extract the first N rows from grouped data. This is very practical in many data analysis scenarios.

MySQL 8 and above provide three commonly used window functions: ROW_NUMBER, RANK and DENSE_RANK. These functions assign a numerical value to each row, allowing us to select the first few rows in each grouping.

Sample data:

Suppose we have the following data set:

pkid	catid	value
1	p01	100
2	p01	90
3	p01	90
4	p01	80
5	p01	80
6	p01	80
7	p01	70
8	p01	60
9	p01	50
10	p01	40

Comparison of the output results of the three window functions:

pkid	catid	value	row_number	rank	dense_rank
1	p01	100	1	1	1
2	p01	90	2	2	2
3	p01	90	3	2	2
4	p01	80	4	4	3
5	p01	80	5	4	3
6	p01	80	6	4	3
7	p01	70	7	7	4
8	p01	60	8	8	5
9	p01	50	9	9	6
10	p01	40	10	10	7

Function description:

• ROW_NUMBER: Assigns a unique sequential number to each row in each group, starting with 1. For example, catid is a p01 whose first 5 rows have row_number values ​​from 1 to 5.

• RANK: Assigns a rank to each row in each grouping, with rows of the same value having the same rank. For example, the first 5 rows where catid is p01 have rank values ​​from 1 to 5 (even if some values ​​are repeated).

• DENSE_RANK: Similar to RANK, but rows with the same value are ranked consecutively without gaps. For example, the first 5 rows where catid is p01 have dense_rank values ​​from 1 to 5 (even if some values ​​are repeated).

Choose the appropriate window function:

Which window function you choose depends on your specific needs and your definition of "first N rows". If you need each row to have a unique ranking, use ROW_NUMBER; if you allow rows with the same value to have the same rank, use RANK; if you need consecutive rankings, use DENSE_RANK.

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