Catch unique index violation and raise_application_error - exception

Question

Is it possible to catch unique index violation and raise_application_error. We are trying with a blanket trigger and catching the exceptions, however we are always getting oracle exception.
ORA-00001: unique constraint (TEST_UNIQUE_INDEX) violated
We have a FUNCTION-BASED constraint on the table.
CREATE TABLE TEST_CONSTRAINT(
"ID" NUMBER NOT NULL ENABLE,
"LOCATION" VARCHAR2(20) NOT NULL,
"DEPT" VARCHAR2(20) NOT NULL,
"RECORD" NUMBER NOT NULL)
/
CREATE UNIQUE INDEX TEST_UNIQUE_INDEX ON TEST_CONSTRAINT (
CASE "RECORD" WHEN 1 THEN "LOCATION" ELSE NULL END,
CASE "RECORD" WHEN 1 THEN "DEPT" ELSE NULL END)
/
before update trigger code
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
ERROR_MESSAGE := SQLERRM;
DBMS_OUTPUT.PUT_LINE('SQLERRM '|| ERROR_MESSAGE);
WHEN OTHERS THEN
ERROR_MESSAGE := SQLERRM;
DBMS_OUTPUT.PUT_LINE('SQLERRM '|| ERROR_MESSAGE);
END;
test schema here
Edit 1:
The requirement here is to impose selective uniqueness i.e. only one set of location/dept can be set as a record (being a boolean in app). In all other cases, we have to generate a raise_application_error.

Answer 1

The trigger would be executed BEFORE the update happens. Since there would be no exception during its execution (all it does is print the system date), no exception would be raised. Then, the update is actually executed, and that's when you get the error message from the database.
To catch the exception that occurs because of the UPDATE statement, the UPDATE statement itself must be enclosed within a PL/SQL block, as below.
DECLARE
V_DATE DATE;
ERROR_MESSAGE VARCHAR2(1000);
BEGIN
SELECT SYSDATE INTO V_DATE FROM DUAL;
DBMS_OUTPUT.PUT_LINE('SYSDATE '|| V_DATE);
UPDATE TEST_CONSTRAINT SET RECORD = 1 WHERE ID = 3;
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
ERROR_MESSAGE := SQLERRM;
DBMS_OUTPUT.PUT_LINE('Error occurred. SQLERRM '|| ERROR_MESSAGE);
WHEN OTHERS THEN
ERROR_MESSAGE := SQLERRM;
DBMS_OUTPUT.PUT_LINE('SQLERRM '|| ERROR_MESSAGE);
END;
/
Reference:
Handling PL/SQL Errors on Oracle® Database PL/SQL Language Reference