Oracle 将时间戳转换为日期

Question

I have date like:

2020/05/09T02:40:03
2020/05/16T02:40:03
2020/05/15T02:40:03

I need to convert it into Date format 'dd/mm/yyyy'.

My query

select cast(date_registered as date) as Reg_date from employee

Error:

Literal does not match format string

Note:I have also used TO_Char , TO_Date but not working

Answer 1

First, what you tried to convert to date is not a timestamp , it is a string.

Second, cast(... to date) does not take a format model - it simply relies on your session nls_date_format parameter, which doesn't match the string's date format. (Especially the boilerplate, hard-coded T in the middle, which has no meaning in Oracle.)

You need to_date() with an appropriate format model. Notice the handling of hardcoded string fragments (they appear in double-quotes in the format model).

select to_date(date_registered, 'yyyy/mm/dd"T"hh24:mi:ss') as reg_date 
from   employee;

Answer 2

In Oracle, a date data type is always stores as 7-bytes consisting of century, year-of-century, month, day, hours, minutes and seconds; so, asking to convert something that has year-to-seconds components to a date does not require you to do anything:

SELECT date_registered AS reg_date FROM employee

Now, if you are storing date_registered as a string data type rather than as a date data type then you need to convert using TO_DATE :

SELECT TO_DATE( date_registered, 'YYYY-MM-DD"T"HH24:MI:SS' ) AS reg_date
FROM   employee

However, you should not do this and you should fix the underlying problem that you are storing dates as strings and not dates. You can solve this by converting the data type of the column:

ALTER TABLE employee ADD ( date_registered2 DATE );
UPDATE employee
  SET date_registered2 = TO_DATE( date_registered, 'YYYY-MM-DD"T"HH24:MI:SS' );
ALTER TABLE employee DROP COLUMN date_registered;
ALTER TABLE employee RENAME COLUMN date_registered2 TO date_registered;

(Note: The queries below assume that you have "fixed" the table so that the date_registered column has the date data type; if you do not want to do this then you will need to include the TO_DATE conversion as well.)

If you want the output to be a date where the time component is truncated back to midnight then:

SELECT TRUNC( date_registered ) AS reg_date
FROM   employee

(Note: The displayed date will still have hours, minutes and seconds components but they will all be zero after being passed through TRUNC .)

If you want it in a specific format, without the time component, then you need to output it as a formatted string (since the date data type does not have an associated format) using TO_CHAR :

SELECT TO_CHAR( date_registered, 'DD/MM/YYYY' ) AS reg_date
FROM   employee

db<>fiddle here