convert into date format oracle - sql

Question

I have a field c_days in the table my_table that accepts numeric values ​​from 1 to 31.
In this field, numbers from 1 to 9 are single digits.
I am writing a condition, if c_day is greater than today, then you need to display c_day in the to_date date format, if less, then display c_day in the date format, only the next month.
For example, c_day is 14, and today we have the 8th number, so you need to display the date 14.02.2023, If c_day is equal to 5, then you need to display the date of the next month 05.03.2023
I did something like this:
SELECT
C_DAY,
CASE
WHEN C_DAY >= TO_CHAR(SYSDATE, 'DD') THEN
TO_DATE(C_DAY || '.' || TO_CHAR(SYSDATE, 'MM') || '.' || TO_CHAR(SYSDATE, 'YYYY'), 'dd.mm.yyyy')
WHEN C_DAY < TO_CHAR(SYSDATE, 'DD') THEN
TO_DATE(C_DAY || '.' || TO_CHAR(SYSDATE, 'MM') || '.' || TO_CHAR(SYSDATE, 'YYYY'), 'dd.mm.yyyy')
WHEN C_DAY IS NULL THEN
null
END AS new_field
FROM my_table
The problem is that the end result is not converted to the date format, I thought it's cause of that the dates can be displayed as 1.03.2023, 7.03.2023, so i tried convert it into
TO_CHAR(C_DAY, 'fm00')
and did this:
SELECT
C_DAY,
CASE
WHEN TO_CHAR(C_DAY, 'fm00') >= TO_CHAR(SYSDATE, 'DD') THEN
TO_DATE(TO_CHAR(C_DAY, 'fm00') || '.' || TO_CHAR(SYSDATE, 'MM') || '.' || TO_CHAR(SYSDATE, 'YYYY'), 'dd.mm.yyyy')
WHEN TO_CHAR(C_DAY, 'fm00') < TO_CHAR(SYSDATE, 'DD') THEN
TO_DATE(TO_CHAR(C_DAY, 'fm00') || '.' || TO_CHAR(SYSDATE, 'MM') || '.' || TO_CHAR(SYSDATE, 'YYYY'), 'dd.mm.yyyy')
WHEN C_DAY IS NULL THEN
null
END AS new_field
FROM my_table
But its not even working, it shows ora error

Answer 1

You can do it without any string-to-date (or vice-versa) conversions using:
SELECT C_DAY,
LEAST(
ADD_MONTHS(
TRUNC(SYSDATE, 'MM'),
CASE WHEN c_day <= EXTRACT(DAY FROM SYSDATE) THEN 0 ELSE 1 END
) + c_day - 1,
LAST_DAY(
ADD_MONTHS(
TRUNC(SYSDATE, 'MM'),
CASE WHEN c_day <= EXTRACT(DAY FROM SYSDATE) THEN 0 ELSE 1 END
)
)
) AS new_field
FROM my_table
Which, for the sample data:
CREATE TABLE my_table(c_day) AS
SELECT LEVEL FROM DUAL CONNECT BY LEVEL <= 31;
Outputs:
C_DAY
NEW_FIELD
1
2023-02-01 00:00:00
2
2023-02-02 00:00:00
3
2023-02-03 00:00:00
4
2023-02-04 00:00:00
5
2023-02-05 00:00:00
6
2023-02-06 00:00:00
7
2023-02-07 00:00:00
8
2023-02-08 00:00:00
9
2023-03-09 00:00:00
10
2023-03-10 00:00:00
11
2023-03-11 00:00:00
12
2023-03-12 00:00:00
13
2023-03-13 00:00:00
14
2023-03-14 00:00:00
15
2023-03-15 00:00:00
16
2023-03-16 00:00:00
17
2023-03-17 00:00:00
18
2023-03-18 00:00:00
19
2023-03-19 00:00:00
20
2023-03-20 00:00:00
21
2023-03-21 00:00:00
22
2023-03-22 00:00:00
23
2023-03-23 00:00:00
24
2023-03-24 00:00:00
25
2023-03-25 00:00:00
26
2023-03-26 00:00:00
27
2023-03-27 00:00:00
28
2023-03-28 00:00:00
29
2023-03-29 00:00:00
30
2023-03-30 00:00:00
31
2023-03-31 00:00:00
fiddle

Answer 2

Here's one option:
SQL> alter session set nls_date_format = 'dd.mm.yyyy';
Session altered.
SQL> WITH
2 my_table (c_day)
3 AS
4 (SELECT 14 FROM DUAL
5 UNION ALL
6 SELECT 5 FROM DUAL
7 UNION ALL
8 SELECT 30 FROM DUAL)
9 SELECT c_day,
10 TO_DATE (
11 LPAD (LEAST (c_day, TO_CHAR (LAST_DAY (SYSDATE), 'dd')), 2, '0')
12 || '.'
13 || TO_CHAR (
14 CASE
15 WHEN c_day < TO_CHAR (SYSDATE, 'dd')
16 THEN
17 ADD_MONTHS (SYSDATE, 1)
18 ELSE
19 SYSDATE
20 END,
21 'mm.yyyy'),
22 'dd.mm.yyyy') AS result
23 FROM my_table;
C_DAY RESULT
---------- ----------
14 14.02.2023
5 05.03.2023
30 28.02.2023
SQL>

Answer 3

As far as I understand your question, one solution could be this one
SELECT
C_DAY,
CASE
WHEN C_DAY >= TO_CHAR(SYSDATE, 'DD') THEN
ADD_MONTHS(TO_DATE(TO_CHAR(SYSDATE, 'YYYY-MM-')||C_DAY , 'YYYY-MM-DD'), 1)
WHEN C_DAY < TO_CHAR(SYSDATE, 'DD') THEN
TO_DATE(TO_CHAR(SYSDATE, 'YYYY-MM-')||C_DAY , 'YYYY-MM-DD')
END AS new_field
FROM my_table
The ADD_MONTHS function works as this:
If date is the last day of the month or if the resulting month has fewer days than the day component of date, then the result is the last day of the resulting month. Otherwise, the result has the same day component as date.