Oracle SQL 日期比较

Question

why if date1 is of type date with value: 31-DIC-99 00:00:00

this instruction is true

to_char(date1) = to_char(TO_DATE('9999-12-31', 'yyyy-MM-dd'))

and this is false?

date1 = TO_DATE('9999-12-31', 'yyyy-MM-dd')

Thanks

Answer 1

Dates in Oracle have time components -- even if you don't seem them when the values are printed out. When you convert the value to a string with no format specified, then the time component is set to zero.

I would recommend writing the logic as:

 trunc(date1) = date '9999-12-31'

Or, if you prefer:

date1 >= date '9999-12-31'

You are referring to the maximum date value, so there are no dates with larger values. . . but there are larger values on the same date with a time.

I do not recommend converting dates to string, except for output purposes (or if needed for a very specific reason). Date/time functions are usually quite sufficient for operations on date/time values.

Answer 2

Here, Your first condition is true because you are using the TO_CHAR both sides and it will convert both of the dates into the same string. (string comparision)

to_char(date1) = to_char(TO_DATE('9999-12-31', 'yyyy-MM-dd'))

Let's say NLS_DATE_FORAMT is dd.mon.rrrr then your comparison will be

to_char(date1,'dd.mon.rrrr') = to_char(TO_DATE('9999-12-31', 'yyyy-MM-dd'),'dd.mon.rrrr')

The second condition is not true as both the sides are different - date1 may have time portion and right side expression do not have it.

date1 = TO_DATE('9999-12-31', 'yyyy-MM-dd')

date1 contains time part so one date will be the same as the second date if they are the same until seconds precision.

You can use trunc to remove the time portion from date1:

trunc(date1) = TO_DATE('9999-12-31', 'yyyy-MM-dd')