DATE_TRUNC

The DATE_TRUNC function in BigQuery will truncate the date to the given date_part.

DATE_TRUNC(date_expression, date_part)

Where date_part can be any of the following:

• DAY

• WEEK

• WEEK(<WEEKDAY>): Truncates date_expression to the preceding week boundary, where weeks begin on WEEKDAY. Valid values for WEEKDAY are SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, and SATURDAY.

• ISOWEEK: Truncates date_expression to the preceding ISO 8601 week boundary. ISOWEEKs begin on Monday. The first ISOWEEK of each ISO year contains the first Thursday of the corresponding Gregorian calendar year. Any date_expression earlier than this will truncate to the preceding Monday.

• MONTH

• QUARTER

• YEAR

• ISOYEAR: Truncates date_expression to the preceding ISO 8601 week-numbering year boundary. The ISO year boundary is the Monday of the first week whose Thursday belongs to the corresponding Gregorian calendar year.

Returns: DATE

SELECT
  DATE_TRUNC(CAST('2021-01-01' AS DATE), MONTH) AS month,
  DATE_TRUNC(CAST('2021-01-01' AS DATE), WEEK) AS week,
  DATE_TRUNC(CAST('2021-01-01' AS DATE), YEAR) AS year

• Always returns a DATE, so even if you are truncating to the YEAR, you'll received back the first day of that year. For that reason, it's often use in conjunction with FORMAT_DATE if you want to just use the year YYYY.

• Be careful when dealing with multiple TRUNCs - as seen in the example above, a date in 2021 can be counted in 2020 if the first day of the week was there. So if you counted by YEAR vs YEAR(WEEK) you would get different results.

DATE_TRUNC is very handy for aggregating your data by a particular date_part, like MONTH. See the example below to see how you can aggregate by MONTH:

SELECT
  SUM(number) AS total,
  DATE_TRUNC(date, MONTH) AS month
FROM
  (
    SELECT
      CAST('2021-02-04' AS DATE) AS date,
      3 AS number
    UNION ALL
(    SELECT
      CAST('2021-02-14' AS DATE) AS date,
      7 AS number)
    UNION ALL
(    SELECT
      CAST('2021-01-04' AS DATE) AS date,
      27 AS number)
  ) AS table_3
GROUP BY
  month

Make sure you've included your DATE_TRUNC column in your GROUP BY!

You'll need to make sure you're using the right function for your data type. To use DATE_TRUNC, you must be working with a DATE, and not a DATETIME, TIMESTAMP, or TIME.

You can use CAST to change your other date types to DATE, or use one of the equivalent functions for other date types like DATETIME_TRUNC.

• DATETIME_TRUNC

• Dates and Times in Standard SQL