Validation of result before executing the query - oracle

Question

When running a stored procedure to fetch some rows, First I want to validate if the query will return a row before sending the result, and second if it is possible to validate without running the same query twice.
I am using a cursor to store the yielded result, So I tried the cursor attribute %ROWCOUNT & %NOTFOUND. But the doesnt quite work. Plus I want to do this without running a loop on the cursor.
procedure MODULE_LIST_GK(p_module_Id IN MODULE_LIST.MODULE_ID% TYPE,
p_Error_Code out nvarchar2,
p_Error_Msg out nvarchar2,
p_Cursor out sys_refcursor) IS
BEGIN
OPEN p_Cursor FOR
SELECT A.MODULE_ID,
A.MODULE_NM,
A.AUTH_STATUS_ID
FROM MODULE_LIST A
WHERE A.MODULE_ID=p_module_Id;
SELECT COUNT(MODULE_ID)
INTO v_row_num
FROM MODULE_LIST A
WHERE A.MODULE_ID=p_module_Id;
IF v_row_num=0 THEN
p_Error_Code := SQLCODE;
p_Error_Msg := 'Does not Exists';
Return;
end IF;
EXCEPTION
WHEN OTHERS THEN
p_error_code:= SQLCODE;
p_error_msg := SQLERRM;
END MODULE_LIST_GK;

Answer 1

Your implementation have several points that could be improved.
First if you expect that for lot of parameters the returned cursor will be empty,
than first check the empty cursor and only after this check open the cursor. You do it vice versa.
How to check if the cursor is empty? Unfortunatelly you must fetch the first row to be able to verify it.
open l_cur for
select id, status from tab where id = p_id;
fetch l_cur into l_id, l_status;
if l_cur%NOTFOUND then
p_Error_Msg := 'Does not Exists';
Return;
end if;
This check is far more effective that the often used count(*) as it is considering only the first (few) rows and not counting all rows in the cursor.
If the check fails you are ready, othervise simple open the cursor and return it.
open l_cur for
select id, status from tab where id = p_id;
p_Cursor := l_cur;
Two additional thinks come to mind.
You should rething the generall approach if the database is very dynamic. How would you handle the case when other session deletes some row between the check and the second open of the cursor?
Finally consider returning an exception instead of the return code.

Answer 2

In order to know whether a cursor contains rows, you must open it and fetch the first row. Once you've done this, it makes no sense anymore to return that cursor, for the recipient will not be able to fetch that first row, because the cursor already points beyond it.
So, you must select twice. What you'd do is to use ROWNUM or an EXISTS clause here to show the DBMS that you are not interested in any more rows. This can speed up the query extremely.
PROCEDURE module_list_gk(p_module_id IN MODULE_LIST.MODULE_ID%TYPE,
p_error_code OUT NVARCHAR2,
p_error_msg OUT NVARCHAR2,
p_cursor OUT SYS_REFCURSOR) IS
v_count INTEGER;
BEGIN
SELECT COUNT(*)
INTO v_count
FROM module_list
WHERE module_id = p_module_Id
AND ROWNUM = 1;
IF v_count = 0 THEN
p_error_code := 0; -- Or -1403 for NO DATA FOUND if you like
p_error_msg := 'Does not Exists';
RETURN;
END IF;
OPEN p_Cursor FOR
SELECT module_id, module_nm, auth_status_id
FROM module_list
WHERE module_id = p_module_id;
EXCEPTION WHEN OTHERS THEN
p_error_code:= SQLCODE;
p_error_msg := SQLERRM;
END module_list_gk;
SQLCODE for the first query will be 0 by the way (SELECT COUNT(*) returns one row with the number of records found - no error hence). This is why you should decide to either return zero explicitly or some error code, such as -1403.
Here is the same with EXISTS:
BEGIN
SELECT CASE WHEN EXISTS
(
SELECT NULL
FROM module_list
WHERE module_id = p_module_Id
) THEN 1 ELSE 0 END
INTO v_count
FROM DUAL;
IF v_count = 0 THEN