Calculating the Count of Records Across Multiple Tables

In this article, we’ll delve into a complex database query that involves multiple tables. Our goal is to calculate the count of records across different hotels for each date.

Problem Overview

We have three tables: CalendarData, HotelResource, and HotelResourcesBookings. The CalendarData table stores dates, while the HotelResource table contains hotel information. The HotelResourcesBookings table holds booking data with a date and hotel ID.

The problem statement asks us to calculate the count of bookings for each hotel on a specific date.

Database Schema

For the purpose of this example, let’s assume the database schema is as follows:

+---------+----------+------------+
|  Table  | Field    | Data Type   |
+---------+----------+------------+
| Calendar| Date     | datetime    |
| HotelResource| Id       | int         |
|             | Name     | varchar(255)|
| HotelResourcesBookings| Id      | int        |
|           | HotelId | int         |
|           | Date     | datetime    |
+---------+----------+------------+

Solution Overview

To solve this problem, we’ll use a combination of SQL techniques:

1. CROSS JOIN: We’ll join the CalendarData table with the HotelResource table to get all possible combinations of dates and hotels.
2. LEFT JOIN: We’ll then left join the HotelResourcesBookings table to distribute booking data across different hotel IDs for each date.
3. PARTITION BY: Finally, we’ll use a window function (COUNT) with a partition clause to calculate the number of bookings per hotel on each date.

SQL Query

Here’s the SQL query that accomplishes this:

SELECT 
    cd.Date,
    hr.Id AS HotelId,
    hr.Name,
    hrb.Id AS HotelResourceId,
    COUNT(hrb.Id) OVER (PARTITION BY cd.Date, hr.Id) AS Count
FROM 
    CalendarData cd
CROSS JOIN 
    HotelResource hr
LEFT JOIN 
    HotelResourcesBookings hrb
    ON cd.Date = hrb.Date
    AND hr.Id = hrb.HotelId
ORDER BY 
    cd.Date, hr.Id, hrb.Id;

Explanation

Here’s a step-by-step explanation of the query:

1. CROSS JOIN: We start by joining the CalendarData table with the HotelResource table using a CROSS JOIN, which returns all possible combinations of dates and hotels.
2. LEFT JOIN: We then left join the HotelResourcesBookings table based on matching dates and hotel IDs.
3. PARTITION BY: Within the LEFT JOIN clause, we use a window function (COUNT) with a partition clause to count the number of bookings per hotel for each date.

Example Output

The query’s output will be a table with columns Date, HotelId, Name, HotelResourceId, and Count. The Count column represents the total number of bookings for each hotel on each date.

+------------+----------+--------+--------------+-------+
| Date       | HotelId | Name  | HotelResourceId| Count|
+------------+----------+--------+--------------+-------+
| 2022-01-01 |        1 | Hotel A| NULL          |     0 |
| 2022-01-01 |        2 | Hotel B| NULL          |     0 |
| ...        | ...      | ...    | ...            | ...   |
+------------+----------+--------+--------------+-------+

Advantages

The provided SQL query has several advantages:

• Flexibility: The query can handle different database schema variations.
• Scalability: The CROSS JOIN and LEFT JOIN operations make it efficient for handling large datasets.
• Readability: The query’s structure is easy to understand, making it a great example of clear SQL coding.

Conclusion

Calculating the count of records across multiple tables involves a combination of database joins and window functions. By using a CROSS JOIN, LEFT JOIN, and PARTITION BY clause, we can efficiently solve this problem and provide valuable insights into booking data distribution.

Last modified on 2024-03-27