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N034-E2 Tier 3 · Intermediate · easy ecommerce · Brightlane

Return the date produced by subtracting `1` month from `'2024-06-15'`

Part of Date Arithmetic and Intervals in SQL

The problem

Brightlane's subscription billing system computes the start of the previous billing period by subtracting one calendar month from the current period's start date.

Write a query to return the date produced by subtracting 1 month from '2024-06-15'.

Output:

  • A single row with one column, prev_period_start, containing the resulting date.
Schema · ecommerce 5 tables
categories
id integer
name text
parent_id? integer
products
id integer
name text
category_id integer
price numeric
stock_qty integer
attributes? jsonb
order_items
id integer
order_id integer
product_id integer
quantity integer
unit_price numeric
customers
id integer
name text
email text
city? text
country text
created_at timestamptz
is_active boolean
orders
id integer
customer_id integer
ordered_at timestamptz
status text
total_amount numeric
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Solution query
SELECT
  '2024-06-15'::date - INTERVAL '1 month' AS prev_period_start

The shape

Subtracting INTERVAL '1 month' from a date walks the calendar back one month and returns the resulting date. Because June 15 has a matching day in May, the result lands cleanly on May 15, 2024.

Clause by clause

  • SELECT '2024-06-15'::date - INTERVAL '1 month' AS prev_period_start builds the previous billing period's start date. The ::date cast types the literal as a calendar date, the - operator subtracts the duration, and INTERVAL '1 month' names the unit as a calendar month rather than a fixed day count. The alias AS prev_period_start labels the output column for the billing system to consume.

Why INTERVAL '1 month' and not INTERVAL '30 days'

The two intervals are different values. '1 month' is a calendar-month duration: PostgreSQL preserves the day-of-month and changes only the month component. '30 days' is a fixed-day duration: it advances the date by exactly thirty days, regardless of how many days the target month happens to have. Subtracting one month from June 15 returns May 15. Subtracting thirty days from June 15 returns May 16. For billing periods that align to calendar months, the calendar-month spelling is the correct one.

You practiced date - INTERVAL '1 month' — subtract a calendar-month duration; PostgreSQL preserves the day-of-month when the target month has it.

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