SQL Query to Calculate Total Number of Weeks Between Two Dates

Calculating the total number of weeks between two dates is a common requirement in data analysis and reporting. This calculation is important for determining time intervals in weeks, which can be useful in various scenarios, such as tracking project timelines, analysing sales trends, or measuring customer activity over a specific period.

In this article, we will explain how to calculate the total number of weeks between two dates using SQL’s DATEDIFF() function. We will have a complete understanding of how to use DATEDIFF() with practical examples to calculate weekly intervals accurately.

What is the DATEDIFF() Function in SQL?

The DATEDIFF() function in SQL calculates the difference between two dates. It is a flexible function that can compute differences in various units, such as days, weeks, months, or years. When calculating weeks, the function is particularly useful for identifying intervals in terms of whole weeks between two dates.

Syntax

SELECT DATEDIFF(WEEK, start_date, end_date);

Example of Calculating Total Number of Weeks Between Two Dates

Let’s demonstrate how to calculate the total number of weeks between two dates using a practical example. For this purpose, we will create a database and a table to store order details.

Query:

CREATE TABLE demo_orders(
  ORDER_ID INT IDENTITY(1,1) PRIMARY KEY,
  ITEM_NAME VARCHAR(30) NOT NULL,
  ORDER_DATE DATE NOT NULL
);

INSERT INTO demo_orders
VALUES
  ('Maserati', '2007-10-03'),
  ('BMW', '2010-07-23'), 
  ('Mercedes Benz', '2012-11-12'),
  ('Ferrari', '2016-05-09'),
  ('Lamborghini', '2020-10-20');

SELECT * FROM demo_orders;

Output

Calculating Weeks Between Two Dates

This section demonstrates how to calculate the time interval in weeks between two specific dates in a table using the DATEDIFF() function. The query retrieves the order dates for the items "Maserati" and "Ferrari" from the demo_orders table and computes the number of full weeks between these dates.

Query: 

DECLARE 
    @start DATE = (
        SELECT order_date 
        FROM demo_orders 
        WHERE item_name = 'Maserati'
    ),
    @end DATE = (
        SELECT order_date 
        FROM demo_orders 
        WHERE item_name = 'Ferrari'
    );

SELECT DATEDIFF(WEEK, @start, @end) AS number_of_weeks;

Output

Explanation:

The output 442 signifies that there are exactly 442 full weeks between the order dates of "Maserati" (2007-10-03) and "Ferrari" (2016-05-09). This calculation excludes partial weeks and provides an accurate weekly interval.

Conclusion

Calculating the total number of weeks between two dates is a straightforward task using SQL’s DATEDIFF() function. By following the steps outlined in this article, we can efficiently compute weekly intervals for various purposes, from business analysis to project management. This method is not only accurate but also flexible, allowing us to adapt it to a wide range of scenarios. Start using the DATEDIFF() function today to streamline our data analysis processes and unlock valuable insights from our datasets.