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N034-M4 Tier 3 · Intermediate · medium ecommerce · Brightlane

Return the number of days between `'2024-01-01'` and `'2024-12-31'`

Part of Date Arithmetic and Intervals in SQL

The problem

Brightlane's data pipeline validates reporting windows by confirming the number of days in a calendar year.

Write a query to return the number of days between '2024-01-01' and '2024-12-31'.

Output:

  • A single row with one column, days_in_year, containing the day count as a number.
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
  '2024-12-31'::date - '2024-01-01'::date AS days_in_year

The shape

DATE - DATE returns an integer day count, and subtracting January 1 from December 31 of the same year returns 365, which matches the day count of a non-leap year. The result is a plain number, suitable for the pipeline's reporting-window check.

Clause by clause

  • SELECT '2024-12-31'::date - '2024-01-01'::date AS days_in_year casts both literals to DATE, subtracts the start of the year from the end, and returns the elapsed days. PostgreSQL counts from one date to the other, exclusive of the start and inclusive of the end. The alias AS days_in_year names the result column.

The trap

365 is the number of days elapsed between January 1 and December 31, not the number of distinct calendar dates in the year. The year 2024 contains 366 distinct dates because it is a leap year, but the subtraction returns 365 because that is how many full days separate the two endpoints. A reader expecting "days in the year" to mean "count of dates that fall in the year" would read this result as off by one. The two questions are different. DATE - DATE answers the elapsed-days question. Counting distinct dates in a range needs a different construction.

You practiced date - date over a full calendar year — the result reflects elapsed days from start to end, not a count of distinct dates spanned.

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