Trunc truncates a date or number and returns the specified value.

Trunc( Number, Num_digits) number needs to be rounded to the end. Num_digits is used to specify the number to take an integer. The default value of num_digits is 0.

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1. Select trunc (sysdate) from dual -- 2011-3-18 today's date is 2011-3-18
2. Select trunc (sysdate, 'mm') from dual -- 2011-3-1 returns the first day of the current month.
3. Select trunc (sysdate, 'yy') from dual -- 2011-1-1 return the first day of the current year
4. Select trunc (sysdate, 'dd') from dual -- 2011-3-18 return current year month day
5. Select trunc (sysdate, 'yyyy') from dual -- 2011-1-1 return the first day of the current year
6. Select trunc (sysdate, 'D') from dual -- 2011-3-13 (Sunday) returns the first day of the current week
7. Select trunc (sysdate, 'hh') from dual -- 2011-3-18 14:00:00 current time is
8. Select trunc (sysdate, 'mi') from dual -- 14:41:00 current minute
The trunc () function does not have second precision.
********************/
/*
Trunc (number, num_digits)
Number.
Num_digits is used to specify the number to take an integer. The default value of num_digits is 0.
When trunc () function is intercepted, No rounding is performed.
*/
9. Select trunc (123.458) from dual -- 123
10. Select trunc (123.458, 0) from dual -- 123
11. Select trunc (123.458, 1) from dual -- 123.4
12. Select trunc (123.458,-1) from dual -- 120
13. Select trunc (123.458,-4) from dual -- 0
14. Select trunc (123.458, 4) from dual -- 123.458
15. Select trunc (123) from dual -- 123
16. Select trunc (123) from dual --
17. Select trunc (123,-1) from dual -- 120

Query data of a day?
Select * From table_name where trunc (Date Field) = to_date ('2017-05-02 ', 'yyyy-mm-dd ');

Trunc truncates a date or number and returns the specified value.