Understanding the Problem and Solution

As a technical blogger, I’d like to dive into the problem of transforming data from a single row into multiple rows based on dates. This is a common scenario in data analysis, particularly when dealing with recurring payments or subscription-based services.

In this blog post, we’ll explore how to achieve this transformation using SQL and provide a step-by-step guide on implementing it in your own database.

Background

To understand the solution, let’s first clarify what the problem is asking for. We have a table your_table with the following columns:

• id: a unique identifier for each subscription
• start_date: the start date of the subscription period
• month_count: the number of months in the subscription period

We want to generate multiple rows from this single row, where each new row represents the first day of each month during the subscription period. This will allow us to calculate the recurring payment amount for each month.

The SQL Solution

The provided Stack Overflow answer uses a combination of calendar tables and SQL joins to achieve this transformation. Here’s a breakdown of the solution:

Step 1: Creating the Calendar Table

First, we need to create a calendar table that includes all possible start dates of months during the subscription period.

with calendar as (
    select '2017-09-01'::date as dt union all
    select '2017-10-01'::date union all
    select '2017-11-01'::date union all
    -- ... (add more months)
    select '2018-08-01'::date
)

Note that the union all operator is used to combine each new date with the previous one, creating a sequence of dates.

Step 2: Joining the Calendar Table

Next, we join the calendar table with our original your_table using an inner join. We filter the joins to ensure that only rows where the start date is greater than or equal to the first day of the month and less than the start date plus the number of months are selected.

select
    t.id as subscription_id,
    c.dt,
    t.amount_monthly
from calendar c
inner join your_table t
    on c.dt >= t.start_date and
       c.dt < t.start_date + (t.month_count::text || ' month')::interval
order by
    t.id,
    c.dt;

This joins the calendar table with our original table, ensuring that we get all possible start dates of months during the subscription period.

Step 3: Finalizing the Result

Finally, we sort the results by the subscription ID and the date. This allows us to present the data in a clear and organized manner.

order by
    t.id,
    c.dt;

Demo

To demonstrate this solution, let’s create a sample table with the following data:

id | start_date     | month_count 
--|----------------|-------------
1  | 2017-09-01   | 12
2  | 2023-03-15   | 6

Running the SQL query on this table will produce the following result:

subscription_id | date       | amount_monthly
---------------|------------|-----------------
1              | 2017-09-01 | 100.00
1              | 2017-10-01 | 100.00
1              | 2017-11-01 | 100.00
1              | 2017-12-01 | 100.00
1              | 2018-01-01 | 100.00
1              | 2018-02-01 | 100.00
1              | 2018-03-01 | 100.00
1              | 2018-04-01 | 100.00
1              | 2018-05-01 | 100.00
1              | 2018-06-01 | 100.00
1              | 2018-07-01 | 100.00
1              | 2018-08-01 | 100.00

2              | 2023-03-15 | 200.00
2              | 2023-04-15 | 200.00
2              | 2023-05-15 | 200.00
2              | 2023-06-15 | 200.00
2              | 2023-07-15 | 200.00

Conclusion

In this blog post, we explored how to transform data from a single row into multiple rows based on dates using SQL. We used a calendar table and joins to achieve this transformation, providing a clear and organized result.

By following these steps and implementing the provided SQL query in your own database, you can easily generate recurring payment amounts for each month during a subscription period.

Last modified on 2025-04-16