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N033-M3 Tier 3 · Intermediate · medium ecommerce · Brightlane

Return the day-of-week number for that timestamp

Part of Date Truncation and Extraction in SQL

The problem

Brightlane's workforce analytics platform records which day of the week each support ticket was submitted. A ticket was logged at '2024-03-15 14:32:07'.

Write a query to return the day-of-week number for that timestamp.

Output:

  • A single row with one column, day_of_week, containing the day number where Sunday is 0 and Saturday is 6.
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
  EXTRACT(
    dow
    FROM
      '2024-03-15 14:32:07'::TIMESTAMP
  ) AS day_of_week

The shape

EXTRACT(dow FROM ...) pulls the day-of-week index out of a datetime and returns a number between 0 and 6. The March 15 timestamp, a Friday, returns 5. The dow field is the standard way to ask which weekday a datetime falls on.

Clause by clause

  • SELECT EXTRACT(dow FROM '2024-03-15 14:32:07'::timestamp) evaluates the extraction against a single literal. The ::timestamp cast resolves the string as a timezone-naive datetime. The day-of-week index is the same regardless of the time-of-day component, so 14:32:07 could be any time during March 15 and the result would still be 5.
  • AS day_of_week labels the output as the index the support ticket analytics consumes.

The trap

PostgreSQL numbers Sunday as 0 and Saturday as 6. Analysts used to systems where Monday is 0 (ISO 8601) or where Sunday is 1 (spreadsheet tools) will read this index off by one and misclassify weekend tickets as weekday tickets, or vice versa. The off-by-one bug is silent: every value is in the right range, every row gets a label, and the misclassification only surfaces when someone compares a count against a known weekly pattern. The fix is to know the convention before consuming the value: PostgreSQL uses dow = 0 for Sunday, full stop. A separate field, isodow, returns 1 through 7 with Monday as 1 for code that needs the ISO convention.

You practiced EXTRACT(dow FROM ...) — PostgreSQL numbers days from Sunday=0 through Saturday=6, so consumers expecting Monday=0 need an offset.

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