The following code gives 244 rows as the result commenting out 2 checks. However, even when I use NVL for the NULL value in group_ofstudy (--AND C1_REC.group_ofstudy = C3_REC.value), I get no rows. Any idea how I can sort this out?

Thanks.

Code:
 

SELECT
	C1_REC.fk_employee_id EMPID_1,
	C2_REC.fk_employee_id EMPID_2,
	C1_REC.fk_exam_level EX_1,
	C4_REC.value1 EX_2,
	C1_REC.fk_qualification QU_1,
	C4_REC.value2 QU_2,
	C1_REC.group_ofstudy GRP_1,
	C3_REC.value GRP_2,
	C1_REC.institution INS_1,
	C3_REC.value INS_2,
	C1_REC.month_of_passing MON_1,
	C2_REC.month_of_passing MON_2,
	C1_REC.year_of_passing YR_1,
	C2_REC.year_of_passing YR_2,
	C1_REC.grade GR_1,
	C2_REC.grade GR_2
FROM
	u_port.d_hris_academic C1_REC,
	d_sip_personal_academic C2_REC,
	d_sip_config_single C3_REC,
	d_sip_config_double C4_REC
WHERE 
	C1_REC.fk_employee_id = C2_REC.fk_employee_id 
	AND C1_REC.fk_exam_level = C4_REC.value1
	AND C1_REC.fk_qualification = C4_REC.value2
	AND C4_REC.double_config_id = C2_REC.fk_qualif_double_config_id
--	AND C1_REC.group_ofstudy = C3_REC.value
--	AND C3_REC.single_config_id = C2_REC.fk_course_single_config_id
	AND C1_REC.institution = C3_REC.value
	AND C3_REC.single_config_id = C2_REC.fk_inst_single_config_id
	AND C1_REC.month_of_passing = C2_REC.month_of_passing
	AND C1_REC.year_of_passing = C2_REC.year_of_passing
	AND C1_REC.grade = C2_REC.grade
ORDER BY 1,2;