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N033-M1 Tier 3 · Intermediate · medium ecommerce · Brightlane

Return the week boundary produced by truncating the timestamp `'2024-03-15 14:32:07'` to week precision

Part of Date Truncation and Extraction in SQL

The problem

Brightlane's weekly engagement digest groups user activity to the start of each calendar week.

Write a query to return the week boundary produced by truncating the timestamp '2024-03-15 14:32:07' to week precision.

Output:

  • A single row with one column, week_start, typed as a timezone-naive timestamp.
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
  DATE_TRUNC('week', '2024-03-15 14:32:07'::TIMESTAMP) AS week_start

The shape

DATE_TRUNC('week', ...) rounds a datetime down to the Monday that starts its ISO calendar week and returns a datetime. The March 15 timestamp, a Friday, lands on '2024-03-11 00:00:00', the Monday three days earlier. That Monday boundary is what makes every weekday inside the same week collapse to a shared value.

Clause by clause

  • SELECT DATE_TRUNC('week', '2024-03-15 14:32:07'::timestamp) evaluates the truncation against a single literal. The ::timestamp cast resolves the string as a timezone-naive datetime, and the truncation preserves the input type, so the result is also a timestamp. The time-of-day component on the input is discarded; only the calendar position matters for finding the week boundary.
  • AS week_start labels the column as the start-of-week value the engagement digest groups around.

The trap

PostgreSQL's week starts on Monday, not Sunday. A learner expecting the Sunday-based convention used by spreadsheet tools and some calendar apps will see the truncation land one day later than they expected and misread it as a bug. The Monday-start rule is the ISO standard and is fixed: there is no field name that flips DATE_TRUNC('week', ...) to a Sunday start. Any consumer that needs a Sunday-start week has to compute the offset separately, not rely on the field name.

You practiced DATE_TRUNC('week', ...) — every datetime within the same calendar week resolves to the same Monday-start boundary.

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