Oracle 11g Release 2

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SELECT TRUNC(sysdate, 'MM') FROM DUAL;

TRUNC(SYSDATE,'MM')

01-APR-19

SELECT TRUNC(sysdate) - TO_NUMBER(TO_CHAR(sysdate,'DD')) + 1 FROM dual

TRUNC(SYSDATE)-TO_NUMBER(TO_CHAR(SYSDATE,'DD'))+1

01-APR-19

SELECT ADD_MONTHS((LAST_DAY(sysdate)+1),-1) FROM DUAL;

ADD_MONTHS((LAST_DAY(SYSDATE)+1),-1)

01-APR-19