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Rounding/truncating timestamps are especially useful when you're grouping by time. The Redshift function you need here is TO_CHAR():
TO_CHAR()
select to_char(sysdate, 'YYYY-MM-DD HH24:MI');
Truncating to date or year is easier because you can just use the TRUNC() and EXTRACT() functions, respectively:
TRUNC()
EXTRACT()
select trunc(sysdate); select extract(year from sysdate);
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