Create function that checks if ID is unique - function

Question

I want to make a function that checks if ID that i will insert is unique.
This is what i did:
-- the function has to return true if ID is unique and false if ID is allready used(not unique)
CREATE OR REPLACE FUNCTION Check_uniqueID
( p_ID IN STORE."ID"%TYPE)
RETURN BOOLEAN
AS
v_IsUnique BOOLEAN := FALSE;
BEGIN
ALTER TABLE STORE
ADD CONSTRAINT check_unique_id UNIQUE (p_ID);
-- how can i programm that he returns true if its unique and false if its not?
v_IsUnique := TRUE;
RETURN v_IsUnique;
END;
Thanks in advance!

Answer 1

The only good practice here is to use unique constraint and sequence. Add constraint (you need to do it once):
ALTER TABLE STORE ADD CONSTRAINT check_unique_id UNIQUE (p_ID);
Create sequence for IDs (once):
create sequence my_sequence;
Use it in INSERT statements:
insert into STORE (id, another_field)
values (my_sequence.nextval, 'some value for another field');
or:
insert into STORE (id, another_field)
select my_sequence.nextval, one_more_field
from some_table;
If you don't want to use a sequence, you need to process exception ORA-00001.

Answer 2

Using an ALTER TABLE in a function just to check if a constraint is being violated is really bad practice. You should have the constraint on the table permanently, and handle the ORA-00001: unique constraint (constraint_name) violated exception should it be thrown. You also seem to have misunderstood the syntax of ALTER TABLE since you're trying to add a constraint on the nonexistant column of STORE.p_ID.
However, if you must use a function to reproduce functionality that the database provides via check constraints (for pre-validation in the UI or similar), this would work:
CREATE OR REPLACE FUNCTION check_uniqueid (p_id IN NUMBER)
RETURN BOOLEAN
AS
li_count PLS_INTEGER;
BEGIN
SELECT COUNT(1)
INTO li_count
FROM store s
WHERE s.id = p_id;
RETURN li_count = 0;
END;

Answer 3

You can use a SELECT query to check whether the value already is stored in the table.