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N033-E1 Tier 3 · Intermediate · easy ecommerce · Brightlane

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

Part of Date Truncation and Extraction in SQL

The problem

Brightlane's reporting pipeline assigns each event to its calendar month for period-based grouping.

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

Output:

  • A single row with one column, month_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('month', '2024-03-15 14:32:07'::TIMESTAMP) AS month_start

The shape

DATE_TRUNC('month', ...) rounds a datetime down to the first instant of its calendar month and returns a datetime. The March 15 timestamp lands on '2024-03-01 00:00:00', which is the period boundary the reporting pipeline keys events against.

Clause by clause

  • SELECT DATE_TRUNC('month', '2024-03-15 14:32:07'::timestamp) evaluates the truncation against a single literal value. The ::timestamp cast resolves the string as a timezone-naive timestamp so DATE_TRUNC returns a timestamp of the same flavor. Without the explicit cast, PostgreSQL would have to infer the input type and the result type would follow whatever inference it landed on.
  • AS month_start labels the output column so the row reads as a domain value rather than a function expression. There is no FROM because the value comes straight from the prompt as a literal.

The trap

The result keeps the full timestamp shape, not just a date. DATE_TRUNC('month', ...) returns '2024-03-01 00:00:00', not '2024-03-01', and its type is timestamp, not date. Any consumer expecting a bare calendar date will need an explicit ::date cast on the output. The function name reads like "give me the month," but what it gives is the start-of-month instant, which is the value that makes two timestamps from anywhere inside March compare as equal.

You practiced DATE_TRUNC('month', ...) — collapse every datetime within a calendar month to the same start-of-month boundary, the standard shape for grouping events by period.

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