Finding Common Closing Days in MySQL Using GROUP BY and HAVING

Learn how to efficiently query common closing days across multiple locations in MySQL using `GROUP BY` and `HAVING` clauses.
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This video is based on the question https://stackoverflow.com/q/65398541/ asked by the user 'Jan Dries' ( https://stackoverflow.com/u/12327869/ ) and on the answer https://stackoverflow.com/a/65398559/ provided by the user 'Gordon Linoff' ( https://stackoverflow.com/u/1144035/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Finding Common Closing Days in MySQL

If you've ever had to query a MySQL database for specific dates that multiple locations have in common, you know how tricky it can be. In this guide, we’re going to tackle a specific scenario involving a table called closing_days. This table tracks the closing days of various locations and is structured with the following columns:

id: The unique identifier for each record.

name: The name of the establishment.

date: The closing date.

kdv_id: An identifier representing each location.

Here's what the data looks like in our closing_days table:

idnamedatekdv_id1foo2021-02-1612bar2021-02-1623baz2021-02-1634qux2021-02-173The Query Challenge

Our goal is straightforward: we want to return only the dates within a certain period where all specified locations share the same closing date. For instance, if you are checking for dates from February 15, 2021, to February 19, 2021, we would like to find returns for kdv_id values 1, 2, and 3.

Given the target period and the example data, the common closing date we want to retrieve is:

2021-02-16.

However, if any location does not have a closing day on a specific date, we should not get that date in our results.

The Solution: Using GROUP BY and HAVING

To achieve this, we will utilize the GROUP BY clause along with the HAVING clause in our SQL query. This allows us to effectively group results by date and then filter these groups based on specific criteria.

Step-by-Step Query Breakdown

Select Dates: Start by selecting the date column from the closing_day table.

Where Clause: Use a WHERE clause to filter based on specific kdv_id values and date range.

Group By: Group the results by the date.

Count Locations: Use the HAVING clause to confirm that the number of unique kdv_ids in the group matches the required number of locations.

Here is how the complete SQL query would look:

[[See Video to Reveal this Text or Code Snippet]]

Key Components Explained

COUNT(DISTINCT kdv_id): This counts how many unique locations have a closing day on each date.

HAVING COUNT(DISTINCT kdv_id) = 3: Ensures that we are only selecting dates for which all three specified locations have a closing day.

Conclusion

By following these steps and using the right SQL constructs, you can effectively query a MySQL database to find common closing days across multiple locations. This approach is efficient and clear, allowing you to get exactly the results you seek without unnecessary complicated logic.

Feel free to use the provided query structure as a foundation for your own database queries, adapting it as necessary for different datasets or locations. Happy querying!

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