ora 取当月第一天

select trunc(sysdate,'month')from dual
trunc会自动载取当月的第一天
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select to_char(trunc(sysdate,'mon'),'yyyymmdd') from dual
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select trunc(sysdate,'mm') from dual;
SELECT trunc(add_months(last_day(sysdate)+1,-1),'dd') from dual
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