MySQL CUME_DIST

The MySQL CUME_DIST function is used to calculate the cumulative distribution of a set of values within a group of rows. It represents the relative position of a specified value within the group, considering the sorted order of the values. The result is a value between 0 and 1, indicating the cumulative distribution of the specified value in the sorted set.

The MySQL CUME_DIST function is commonly used in statistical analysis and ranking tasks. Here are some specific applications:

Identifying percentiles: The cumulative distribution can be used to calculate percentiles, such as the 25th percentile (Q1), 50th percentile (median), and 75th percentile (Q3).

Identifying low-performing or high-performing groups: By analyzing the cumulative distribution, you can identify groups of students or products that have consistently performed better or worse than others.

Comparing cumulative distributions across different groups: This can be useful for analyzing performance trends or identifying potential outliers.

Visualizing data distributions: The cumulative distribution can be represented graphically to provide a clearer understanding of the spread and distribution of data.

Syntax

Here is the syntax for the CUME_DIST function:

CUME_DIST() OVER (
  [PARTITION BY partition_expression, ... ]
  ORDER BY sort_expression [ASC | DESC], ...
)

PARTITION BY: Optional clause that divides the result set into partitions to which the CUME_DIST function is applied independently.
ORDER BY: Specifies the column or columns by which the result set is sorted. The CUME_DIST function considers this order when calculating the cumulative distribution.

Example

Now, let’s look at an example to better understand the usage of the CUME_DIST function. Consider a table named sales:

CREATE TABLE sales (
  product_id INT,
  sale_amount DECIMAL(10, 2)
);

INSERT INTO sales (product_id, sale_amount) VALUES
(1, 100.50),
(1, 150.25),
(1, 200.75),
(2, 50.00),
(2, 75.50),
(3, 120.00),
(3, 180.25);

Now, if you want to calculate the cumulative distribution of the sale amounts within each product group, you can use the CUME_DIST function:

SELECT
  product_id,
  sale_amount,
  CUME_DIST() OVER (PARTITION BY product_id ORDER BY sale_amount) AS cumulative_distribution
FROM
  sales;

In this example, the CUME_DIST function is applied within each product_id partition and orders the rows by the sale_amount. The result will show the cumulative distribution of sale amounts for each product. The output might look like this:

| product_id | sale_amount | cumulative_distribution |
|------------|-------------|-------------------------|
| 1          | 100.50      | 0.3333                  |
| 1          | 150.25      | 0.6667                  |
| 1          | 200.75      | 1.0000                  |
| 2          | 50.00       | 0.5                     |
| 2          | 75.50       | 1.0                     |
| 3          | 120.00      | 0.5                     |
| 3          | 180.25      | 1.0                     |

In this result, the cumulative_distribution column represents the cumulative distribution of the sale amounts within each product group.

The MySQL CUME_DIST function is a powerful window function for understanding data distributions and identifying patterns in numerical data. It plays a crucial role in data analysis and statistical applications within MySQL.