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N034-M2 Tier 3 · Intermediate · medium ecommerce · Brightlane

Return the duration between the placement timestamp `'2024-01-01 09:00:00'` and the shipment timestamp `'2024-03-15 12:00:00'`

Part of Date Arithmetic and Intervals in SQL

The problem

Brightlane's logistics platform measures the elapsed time between when an order was placed and when it shipped.

Write a query to return the duration between the placement timestamp '2024-01-01 09:00:00' and the shipment timestamp '2024-03-15 12:00:00'.

Output:

  • A single row with one column, elapsed_time, typed as an interval.
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 12:00:00'::TIMESTAMP - '2024-01-01 09:00:00'::TIMESTAMP AS elapsed_time

The shape

Subtracting one TIMESTAMP from another returns an INTERVAL that carries both the day count and the hour count between them. Unlike DATE - DATE, which returns a plain integer, timestamp subtraction returns a duration type because the difference can include sub-day components.

Clause by clause

  • SELECT '2024-03-15 12:00:00'::timestamp - '2024-01-01 09:00:00'::timestamp AS elapsed_time casts both literals to TIMESTAMP and subtracts the earlier from the later. PostgreSQL computes the difference and returns 74 days 03:00:00: seventy-four whole days between the calendar positions, plus the three-hour gap between 09:00 and 12:00. The alias AS elapsed_time names the duration column.

Why this returns an INTERVAL and not an integer

The return type of date and timestamp subtraction depends on the input types. DATE - DATE returns an integer because dates have no time-of-day component and the gap can be expressed as a whole-number day count. TIMESTAMP - TIMESTAMP returns an INTERVAL because timestamps carry hours, minutes, and seconds, and a number cannot express that combined gap. The duration type can: it stores days and the sub-day remainder side by side.

You practiced timestamp - timestamp returning an INTERVAL — duration arithmetic produces a duration type, not a number.

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