原文地址：http://impala.apache.org/docs/build/html/topics/impala_datetime_functions.html

TRUNC(TIMESTAMP / DATE ts, STRING unit)

Purpose: Returns the ts truncated(截断的) to the unit specified.

Argument: The unit argument is not case-sensitive. This argument string can be one of:

Unit Supported for TIMESTAMP ts Suppport for
DATE ts		 Description
SYYYY / YYYY / YEAR
SYEAR / YYY / YY / Y		 Y Y Year
Q Y Y Quarter(季度)
MONTH / MON
MM / RM		 Y Y Month
WW Y Y The most recent date that is the same day of the week as the first day of the year
W Y Y The most recent date that is the same day of the week as the first day of the month
DDD / DD / J Y Y Day
DAY / DY / D Y Y Starting day of the week (Monday)
HH / HH12 / HH24 Y N Hour. A TIMESTAMP value truncated to the hour is always represented in 24-hour notation, even for the HH12 argument string.
MI Y N Minute

Added in: The ability to truncate numeric values is new starting in Impala 2.10.

Usage notes:
The TIMESTAMP form is typically used in GROUP BY queries to aggregate results from the same hour, day, week, month, quarter, and so on. You can also use this function in an INSERT ... SELECT into a partitioned table to divide TIMESTAMP values into the correct partition.

Return type:

  • TIMESTAMP if the first argument, ts, is TIMESTAMP.
  • DATE if the first argument, ts, is DATE.

Example:

  1. SELECT TRUNC(DATE'2019-05-08','YEAR')  -- 2019-01-01
  3. SELECT TRUNC(DATE'2019-05-08', 'QUARTER')  -- 2019-04-01
  5. --每月1号
  6. SELECT TRUNC(NOW(), 'MM')  --    2021-04-01 00:00:00