SQL window functions are strong instruments for complex data manipulation and analysis. The four primary window functions in T-SQL are ROW_NUMBER, RANK, DENSE_RANK, and NTILE. Within result sets, these functions aid in ranking, sorting, and organizing data. Let's examine each function in more detail, learning about its purpose and practical applications.

To comprehend the ideas, let's look at the table below.

CREATE TABLE StudentMarks (
    StudentID INT,
    Subject VARCHAR(50),
    Marks INT
);

Adding a few sample records in the above table.

1. ROW_NUMBER

A window function called ROW_NUMBER uses the given ordering to assign a distinct sequential number to each row inside a partition of a result set. It starts at 1 and generates a new number for every row, without any pauses.

Use Cases: ROW_NUMBER is frequently used to filter the top N rows in a partition, discover duplicates, and create pagination.

Example

SELECT *,
       ROW_NUMBER() OVER (ORDER BY Marks DESC) AS RowNumber
FROM StudentMarks;

It will return the output.

2. RANK

Another window method called RANK uses a given ordering to give each row inside a partition of a result set a distinct rank. When gaps occur in the ranking sequence, it leaves them there and gives the same rank to rows with equal values.

Use Cases: When ranking is permitted to have ties, like in the case of rating students based on exam results, RANK is frequently employed.

Another window method called RANK uses a given ordering to give each row inside a partition of a result set a distinct rank. When gaps occur in the ranking sequence, it leaves them there and gives the same rank to rows with equal values.

Use Cases: RANK is often used when ranking is allowed to contain ties, such as when grading students according to exam scores.

Example:

SELECT *,
       RANK() OVER (ORDER BY Marks DESC) AS Rank
FROM StudentMarks;

It will return the output.

3. DENSE_RANK
DENSE_RANK is similar to RANK but differs in that it assigns consecutive ranks to rows with equal values, without leaving gaps in the ranking sequence. It ensures that ranks are assigned in a continuous, sequential manner.

Use Cases: DENSE_RANK is preferred when consecutive ranking without gaps is desired, such as ranking products by sales performance.

Example
SELECT *,
       DENSE_RANK() OVER (ORDER BY Marks DESC) AS DenseRank
FROM StudentMarks;

It will return the output.

4. NTILE
NTILE is a window function that divides the result set into a specified number of roughly equal-sized buckets or partitions, assigning each row to one of these buckets. The function ensures that the size difference between buckets is minimized.

Use Cases: NTILE is commonly used for data segmentation and percentile calculations, such as dividing customers into groups based on their income.

Example
SELECT *,
       NTILE(4) OVER (PARTITION BY SUBJECT ORDER BY Marks DESC) AS Student_Group
FROM StudentMarks;

It will return the output.
studentID   Subject

It will return the output.

In summary
With SQL Server, window functions like ROW_NUMBER, RANK, DENSE_RANK, and NTILE give analysts and developers strong capabilities for data analysis, ranking, and partitioning. Through the appropriate utilization of these functionalities, users can accomplish intricate analytical tasks, acquire a deeper understanding of their data, and retrieve important information from their databases. By being aware of the subtleties and functionalities of each window function, SQL practitioners can fully utilize T-SQL for complex data manipulation and analysis activities.

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