Advanced Techniques in SQL

Challenge: Using Window Function

Let's consider the bank_accounts table we have used in the previous sections:

account_number	account_holder	balance
789	Alice Johnson	3500.00
101	Bob Brown	4500.00
202	Emily Jones	6000.00
303	Michael Davis	800.00
404	Sarah Wilson	12000.00
505	David Taylor	300.00
606	Emma Thomas	7500.00
707	Daniel Martinez	1000.00
808	Olivia Rodriguez	900.00
909	Sophia Lee	2500.00
123	John Doe	5000.00
456	Jane Smith	2000.00

Let's assume we want to add information to the table about which class each user belongs to based on their account balance.

The classes will be determined as follows:

A person whose account balance is in the bottom 25% will belong to the forth class;
A person whose account balance is between 25% and 50% will belong to the third class;
A person whose account balance is between 50% and 75% will belong to the second class;
A person whose account balance is in the top 25% will belong to the first class.

To solve this problem, we can use a window function called NTILE().
The NTILE() function is a window function that divides the rows in an ordered partition into a specified number of roughly equal groups, or "tiles".
It assigns each row a group number, starting at 1, based on the group it falls into.

Note
In the example, we don't need to specify a PARTITION BY clause because we want to apply the NTILE() function to the entire result set, not to specific subsets or partitions of the data.
The ORDER BY clause is sufficient as it defines the order in which the rows are distributed into tiles, ensuring that the grouping is based on the sorted order of the salaries across the entire table.
If we wanted to calculate tiles within specific groups (e.g., departments), we would use PARTITION BY.

Task

Your task is to apply NTILE() function to get indormation about class of each account holder.

You have to:

Specify number of classes as the argument of the function.
Provide ordering by account balance to determine class.
As we assume, that the first class is the class with the highest balance and so on, we have to order balances in descending order.

Everything was clear?

Section 3. Chapter 5