Raise exception in stored procedure based on view entries - sql

Question

I have a stored procedure. I would like to implement the below logic, which I have written in pseudocode.
If the below query has one of more entries:
SELECT
NULL
FROM
table1
WHERE
condition
GROUP BY
column
HAVING
COUNT(1) > 1
UNION ALL
SELECT
NULL
FROM
table1 a
WHERE
condition
AND EXISTS (
SELECT
NULL
FROM
table2 b
WHERE
condition
);
Then raise an exception and stop the stored procedure.

Answer 1

Here is an example of raising an exception if a particular value is found from a query:
declare
somevar dual.dummy%type;
begin
select 'Y' into somevar
from dual;
if somevar = 'Y' then
raise_application_error(-20123, 'Hull breach on deck 15. Abandon ship.');
end if;
end;
The "select from dual" can be any query, so feel free to substitute your unions and counts (though we should really stick to the standard count(*), not count('Dracula') etc).

Answer 2

Let's do this with the sample emp/dept schema - just plug in your own statement for your use case. You do need to declare since in pl/sql you cannot "just select". You always need to select into a variable. I usually just select the number 1 into a dummy variable of type number. The trick is to raise the exception after the SELECT INTO and do nothing on NO_DATA_FOUND.
You can use named exceptions to distinguish different cases but since a no data found will throw an exception you have to do each of the cases in its own block. The cleanest is to handle all named exceptions in the final exception block.
DECLARE
l_dummy NUMBER;
king_exists EXCEPTION;
dave_exists EXCEPTION;
BEGIN
BEGIN
SELECT 1 INTO l_dummy FROM emp WHERE ename = 'DAVE';
RAISE dave_exists;
EXCEPTION WHEN NO_DATA_FOUND THEN
NULL;
END;
BEGIN
SELECT 1 INTO l_dummy FROM emp WHERE ename = 'KING';
RAISE king_exists;
EXCEPTION WHEN NO_DATA_FOUND THEN
NULL;
END;
EXCEPTION
WHEN dave_exists THEN
raise_application_error(-20000,'My expection error message');
WHEN king_exists THEN
raise_application_error(-20001,'King exists');
END;
/