Oracle PL/SQL code return null - sql

Question

I write this function in oracle PL/SQL to return counter of employee in the university based on their sex and collage nationality,sex , grade and qualification ETC.
FUNCTION GET_EMP_COUNT_GENERIC (
P_ORGANIZATION_ID VARCHAR2,
P_GRADE VARCHAR2,
P_SEX VARCHAR2,
P_NAT_TYPE VARCHAR2,
P_DATE DATE,
P_EMP_TYPE VARCHAR2,
P_QUALIFIC_CAT VARCHAR2 )
RETURN NUMBER IS
V_COUNT NUMBER;
BEGIN
SELECT COUNT (EMP.PERSON_ID) INTO V_COUNT
FROM PER_ALL_ASSIGNMENTS_F ASS,
PER_ALL_PEOPLE_F EMP,
HR_ALL_ORGANIZATION_UNITS ORG,
PAY_PEOPLE_GROUPS PPG,
PER_GRADES GRD
WHERE ASS.PERSON_ID = EMP.PERSON_ID
AND ASS.ORGANIZATION_ID = ORG.ORGANIZATION_ID
AND ASS.PEOPLE_GROUP_ID = PPG.PEOPLE_GROUP_ID
AND ASS.GRADE_ID = GRD.GRADE_ID
AND EMP.PERSON_TYPE_ID = 1145
AND (ASS.ORGANIZATION_ID = P_ORGANIZATION_ID OR P_ORGANIZATION_ID IS NULL)
AND (GRD.NAME = P_GRADE OR P_GRADE IS NULL)
AND (EMP.SEX = P_SEX OR P_SEX IS NULL)
AND (PPG.SEGMENT2 = P_NAT_TYPE OR P_NAT_TYPE IS NULL)
AND (PPG.SEGMENT1 =P_EMP_TYPE OR P_EMP_TYPE IS NULL)
AND (XXPNU_HCM_PKG.GET_QUALIFICATION_NAME(EMP.PERSON_ID,EMP.BUSINESS_GROUP_ID)
= P_QUALIFIC_CAT OR P_QUALIFIC_CAT IS NULL)
AND P_DATE BETWEEN ASS.EFFECTIVE_START_DATE
AND ASS.EFFECTIVE_END_DATE
AND P_DATE BETWEEN EMP.EFFECTIVE_START_DATE
AND EMP.EFFECTIVE_END_DATE;
RETURN V_COUNT;
END GET_EMP_COUNT_GENERIC;
But when I call it in query, its return null or zero value??
i dont know what is my mistake
SELECT
XXPNU_HCM_PKG.GET_EMP_COUNT_GENERIC (null,null,'M',null,:P_DATE,'FACULTY',null) as FACULTY_M,
XXPNU_HCM_PKG.GET_EMP_COUNT_GENERIC (null,null,'F',null,:P_DATE,'FACULTY',null) as FACULTY_F
--------------------------------TABLES------------------------------------------
FROM PER_ALL_ASSIGNMENTS_F ASS,
PER_ALL_PEOPLE_F EMP,
HR_ALL_ORGANIZATION_UNITS ORG,
PAY_PEOPLE_GROUPS PPG,
PER_GRADES GRD
--------------------------------- CONDITIONS -----------------------------------
WHERE
ASS.PERSON_ID = EMP.PERSON_ID
AND ASS.ORGANIZATION_ID = ORG.ORGANIZATION_ID
AND ASS.PEOPLE_GROUP_ID = PPG.PEOPLE_GROUP_ID
AND ASS.GRADE_ID = GRD.GRADE_ID
AND EMP.PERSON_TYPE_ID = 1145
--------------------------------THIS YEAR DATE ---------------------------------
AND :P_DATE BETWEEN ASS.EFFECTIVE_START_DATE
AND ASS.EFFECTIVE_END_DATE
AND :P_DATE BETWEEN EMP.EFFECTIVE_START_DATE
AND EMP.EFFECTIVE_END_DATE ;

Answer 1

Your function will return 0 when it finds no rows that match your predicates in the function.
And because you use the function in the select-section of a select-statement it will return NULL when a exception occurs within the function.
Try using an exceptio handler returning -1. Then you know an exception occured.
remenber, an exception is not handled when a function is called within a select statement (in all DML I would say, but I don't know that for sure)