How to use external functions in check constraints

Question

I created my own function:
create or replace
function if_num_get_num (inval in varchar2)
return number
is
dummy number;
Begin
dummy := to_number(inval);
return dummy;
exception
when others then return null;
end;Can I use it in table check constraint?
When I use standard function INSTR everything is OK.
ALTER TABLE A_S
ADD CONSTRAINT A_S_CHK1 CHECK
  (INSTR(NAZWA_ZA, ':') > 0)
ENABLE;but when I try to create it with my function:
ALTER TABLE A_S
ADD CONSTRAINT A_S_CHK1 CHECK
  (IF_NUM_GET_NUM(INSTR(NAZWA_ZA, ':')) > 0)
ENABLE;I get a message "Invalid column IF_NUM_GET_NUM"

Answer 1

Read the restrictions.
>
Restrictions on CHECK Constraints
A CHECK constraint requires that a condition be true or unknown for every row of the table. If a statement causes the condition to evaluate to false, then the statement is rolled back. The condition of a CHECK constraint has these limitations:
* The condition must be a boolean expression that can be evaluated using the values in the row being inserted or updated.
* The condition cannot contain subqueries or sequences.
* The condition cannot include the SYSDATE, UID, USER, or USERENV SQL functions.
* The condition cannot contain the pseudocolumns LEVEL or ROWNUM.
* The condition cannot contain the PRIOR operator.
** The condition cannot contain a user-defined function.*
>
http://download.oracle.com/docs/cd/E11882_01/appdev.112/e17125/adfns_constraints.htm#ADFNS282