check for valid date which is declared in varchar2 - sql

Question

My table looks like below which is declared in VARCHAR2:
YMD
20101010
20101112
20100231
20150101
20160101
I have to check for valid dates and filter future dates from sysdate, which are in valid format.
I write the function as below to check for valid dates:
create or replace FUNCTION VALIDATE_DATE (p_string in string) return date is
begin
return to_date(p_string, 'YYYYMMDD');
exception when others then
begin
return to_date(p_string, 'YYYY-MM-DD');
exception when others then
begin
return to_date(p_string, 'RR-MON-DD');
exception when others then
return null;
end;
end;
end;
and written this query to check for valid dates and replace with null for invalid dates
select ymd, VALIDATE_DATE(ymd) as Valid
from temp
and to check future dates I wrote the following query, but it throws error
ORA-01839
select ymd
from temp
where validate_date(ymd)='YES'
and to_date(ymd,'yyyymmdd')>sysdate
How to check future dates in my table if exists?

Answer 1

I will rather fix the design issue as a permanent fix rather than wasting time on the workaround.
Firstly, NEVER store DATE as VARCHAR2. All this overhead is due to the fact that your design is flawed.
'20100231'
How on earth could that be a valid date? Which calendar has a 31 days in FEBRUARY?
Follow these steps:
Add a new column with DATE DATA TYPE.
Update the new column with date values from the old column using TO_DATE.
Do the required DATE arithmetic on the new DATE column, or handle this in the UPDATE statement in step 2 itself.
Drop the old column.
Rename the new column to the old column.
UPDATE Adding a demo
Setup
SQL> CREATE TABLE t
2 (ymd varchar2(8));
Table created.
SQL>
SQL> INSERT ALL
2 INTO t (ymd)
3 VALUES ('20101112')
4 --INTO t (ymd)
5 -- VALUES ('20100231')
6 INTO t (ymd)
7 VALUES ('20150101')
8 INTO t (ymd)
9 VALUES ('20160101')
10 SELECT * FROM dual;
3 rows created.
SQL>
SQL> COMMIT;
Commit complete.
SQL>
Add new column:
SQL> ALTER TABLE t ADD (dt DATE);
Table altered.
SQL>
DO the required update
SQL> UPDATE t
2 SET dt =
3 CASE
4 WHEN to_date(ymd, 'YYYYMMDD') > SYSDATE
5 THEN NULL
6 ELSE to_date(ymd, 'YYYYMMDD')
7 END;
3 rows updated.
SQL>
SQL> COMMIT;
Commit complete.
SQL>
Let's check:
SQL> SELECT * FROM t;
YMD DT
-------- ---------
20101112 12-NOV-10
20150101 01-JAN-15
20160101
SQL>
Drop the old column:
SQL> ALTER TABLE t DROP COLUMN ymd;
Table altered.
SQL>
Rename the new column to old column name
SQL> ALTER TABLE t RENAME COLUMN dt TO ymd;
Table altered.
SQL>
You have just fixed the issue
SQL> SELECT * FROM t;
YMD
---------
12-NOV-10
01-JAN-15
SQL>