+ date_trunc() function
+ Purpose: Truncates a TIMESTAMP value to the specified precision.
+
+ Unit argument: The unit argument value for truncating
+ TIMESTAMP values is not case-sensitive. This argument string
+ can be one of:
+
+
+ - microseconds
+ - milliseconds
+ - second
+ - minute
+ - hour
+ - day
+ - week
+ - month
+ - year
+ - decade
+ - century
+ - millennium
+
+
+ For example, calling date_trunc('hour',ts) truncates
+ ts to the beginning of the corresponding hour, with
+ all minutes, seconds, milliseconds, and so on set to zero. Calling
+ date_trunc('milliseconds',ts) truncates
+ ts to the beginning of the corresponding millisecond,
+ with all microseconds and nanoseconds set to zero.
+
+
+ The sub-second units are specified in plural form. All units representing
+ one second or more are specified in singular form.
+
+
+
+
+ Although this function is similar to calling TRUNC()
+ with a TIMESTAMP argument, the order of arguments
+ and the recognized units are different between TRUNC()
+ and DATE_TRUNC(). Therefore, these functions are not
+ interchangeable.
+
+
+ This function 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.
+
+
+ Because the return value is a TIMESTAMP, if you cast the result of
+ DATE_TRUNC() to STRING, you will often see zeroed-out portions such as
+ 00:00:00 in the time field. If you only need the individual units such as hour, day,
+ month, or year, use the EXTRACT() function instead. If you need the individual units
+ from a truncated TIMESTAMP value, run the TRUNCATE() function on the
+ original value, then run EXTRACT() on the result.
+
+
+ Return type: timestamp
+
+
+
+ The following examples show how to call DATE_TRUNC() with different unit values:
+
+
+select now(), date_trunc('second', now());
++-------------------------------+-----------------------------------+
+| now() | date_trunc('second', now()) |
++-------------------------------+-----------------------------------+
+| 2017-12-05 13:58:04.565403000 | 2017-12-05 13:58:04 |
++-------------------------------+-----------------------------------+
+
+select now(), date_trunc('hour', now());
++-------------------------------+---------------------------+
+| now() | date_trunc('hour', now()) |
++-------------------------------+---------------------------+
+| 2017-12-05 13:59:01.884459000 | 2017-12-05 13:00:00 |
++-------------------------------+---------------------------+
+
+select now(), date_trunc('millennium', now());
++-------------------------------+---------------------------------+
+| now() | date_trunc('millennium', now()) |
++-------------------------------+---------------------------------+
+| 2017-12-05 14:00:30.296812000 | 2000-01-01 00:00:00 |
++-------------------------------+---------------------------------+
+
+
+
+