date

DATE_TRUNC()

Truncates a date/timestamp down to the start of a given unit.

Description

DATE_TRUNC rounds a date or timestamp down to the beginning of the specified unit, the start of the year, month, day, hour, etc. It's the standard way to bucket rows by time period for grouping (e.g. sales per month).

Syntax

DATE_TRUNC('month', source)
DATE_TRUNC('day', source)

Parameters

Name	Description	Optional
unit	The truncation unit: year, quarter, month, week, day, hour, minute, second.	No
source	The date/timestamp to truncate.	No

Return Type

Returns a TIMESTAMP (or DATE if the input is a date).

Examples

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Group by period

GROUP BY DATE_TRUNC('month', hire_date) gives clean per-month buckets without string formatting.

Common Mistakes

Wrong unit name. Units are engine-specific strings; 'mm' vs 'month' differs by database.
Expecting rounding up. DATE_TRUNC always goes down to the unit start.
Mixing date and timestamp. The result type follows the input; a DATE stays a DATE.