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N034-E3 Tier 3 · Intermediate · easy ecommerce · Brightlane

Return the number of days between the start date `'2024-01-01'` and the end date `'2024-03-15'`

Part of Date Arithmetic and Intervals in SQL

The problem

Brightlane's contract management system reports the number of days a contract has been active.

Write a query to return the number of days between the start date '2024-01-01' and the end date '2024-03-15'.

Output:

  • A single row with one column, days_active, 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
  '2024-03-15'::date - '2024-01-01'::date AS days_active

The shape

Subtracting one DATE from another returns the integer number of days between them. The result is a plain number, not a duration type, which is exactly the day count the contract management report needs.

Clause by clause

  • SELECT '2024-03-15'::date - '2024-01-01'::date AS days_active casts both literals to DATE and subtracts the earlier date from the later one. PostgreSQL counts the elapsed days and returns 74. The alias AS days_active names the column for the report.

Why this returns an integer and not an INTERVAL

DATE - DATE is a special case in PostgreSQL's date arithmetic. It produces a plain integer rather than an INTERVAL, because the result is unambiguous: a day count between two calendar dates has no hours, no months, no fractional component. Subtracting two TIMESTAMP values returns an INTERVAL instead, because timestamps can differ by hours and seconds as well as by days, and an INTERVAL is the type that can carry all of those components. The choice of return type follows the precision of the input type.

You practiced date - date returning an integer day count — PostgreSQL produces a plain number for date subtraction, not a duration type.

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