Adventures in Machine Learning

Mastering Data Sorting and Grouping with ROW_NUMBER() and DENSE_RANK()

Using ROW_NUMBER() and DENSE_RANK() to Sort Data: An Overview

When you work with data, one of the things you are likely to do frequently is sort it. Whether you’re organizing customer data in a CRM or analyzing sales data in an Excel spreadsheet, sorting data is a crucial task.

Most often, when you sort data, you’re trying to group it based on specific criteria, such as date, location, or product type. The idea is to isolate individual groups of data and work with them separately.

That’s where functions like ROW_NUMBER() and DENSE_RANK() come in handy. ROW_NUMBER() is a SQL server function that assigns a unique sequential integer to each row in a result set.

When you use ROW_NUMBER() in combination with a PARTITION BY clause, you can create groups of data based on a specific column or columns. You can then use the result to select the first row of each group.

DENSE_RANK() is similar to ROW_NUMBER() with the key difference that the function assigns a rank or grouping to each row in the result set. In other words, it counts not only the unique values but also assigns a unique rank to each value in the result set.

For example, if the result set contained five unique values, the first value would have a rank of one, the second value would have a rank of two, and so on. When you use DENSE_RANK() in combination with a PARTITION BY clause, you can create groups of data based on a specific column or columns, like you can with ROW_NUMBER().

However, DENSE_RANK() can also return more than one row per group. In contrast, ROW_NUMBER() will only return one row per group.

In the following sections, we’ll explore how to use ROW_NUMBER() and DENSE_RANK() to sort data, create groups, and select specific rows from each group.

Using ROW_NUMBER() to Display the First Row of Each Group

To use ROW_NUMBER() to display the first row of each group in a result set, you need to follow these steps:

1. Use the SELECT statement to retrieve the data you want to sort.

2. Use the ROW_NUMBER() function to assign a unique integer to each row in the result set.

3. Partition the data using the PARTITION BY clause.

4. Sort the data within each partition using the ORDER BY clause.

5. Use a WHERE clause to select only the first row in each partition.

Here’s an example that demonstrates how you can use ROW_NUMBER() to display the first row of each group in a table named ‘customer_orders’ based on the ‘order_date’ column:

SELECT *

FROM (

SELECT *, ROW_NUMBER() OVER (

PARTITION BY order_date

ORDER BY order_total DESC) AS row_num

FROM customer_orders

) ordered_table

WHERE ordered_table.row_num = 1;

In this example, the SELECT statement retrieves all columns from the table ‘customer_orders’, the ROW_NUMBER() function is used to assign a unique sequential integer to each row. The PARTITION BY clause is used to create groups based on the ‘order_date’ column.

The ORDER BY clause sorts the data within each partition based on the ‘order_total’ column in descending order. Finally, the WHERE clause is used to select only the first row of each group.

Using DENSE_RANK() to Display the Second Row of Each Group

To use DENSE_RANK() to display the second row of each group in a result set, you need to follow these steps:

1. Use the SELECT statement to retrieve the data you want to sort.

2. Use the DENSE_RANK() function to assign a grouping to each row in the result set.

3. Partition the data using the PARTITION BY clause.

4. Sort the data within each partition using the ORDER BY clause.

5. Use a WHERE clause to select only the second row in each partition.

Here’s an example that demonstrates how you can use DENSE_RANK() to display the second row of each group in a table named ‘customer_orders’ based on the ‘order_date’ column:

SELECT *

FROM (

SELECT *, DENSE_RANK() OVER (

PARTITION BY order_date

ORDER BY order_total DESC) AS rank_num

FROM customer_orders

) ordered_table

WHERE ordered_table.rank_num = 2;

In this example, the SELECT statement retrieves all columns from the table ‘customer_orders’, the DENSE_RANK() function is used to assign a grouping to each row where the PARTITION BY clause is used to create groups based on the ‘order_date’. The ORDER BY clause sorts the data within each partition based on the ‘order_total’ column in descending order.

Finally, the WHERE clause is used to select only the second row of each group.

Conclusion

In conclusion, ROW_NUMBER() and DENSE_RANK() are powerful functions that allow you to sort and group data based on specific criteria easily. When you use these functions in conjunction with the PARTITION BY and ORDER BY clauses, you can create groups of data within your result set.

By selecting specific rows within these groups, you can isolate and analyze data based on specific criteria. Whether you’re working with a CRM system, a database management system, or an Excel spreadsheet, using ROW_NUMBER() and DENSE_RANK() functions can make data analysis a lot easier.

In summary, sorting and grouping data is an essential task for any data analyst. ROW_NUMBER() and DENSE_RANK() functions, used in conjunction with the PARTITION BY and ORDER BY clauses, can make data analysis much more manageable.

By assigning unique integers or rankings to each row and partitioning data into groups based on specific columns, these functions allow the isolation and analysis of data based on specific criteria. If you are a data analyst, a CRM user, or work with databases, understanding how to use these functions can lead to improved data analysis skills and more efficient work processes.