SQL Oracle Query - Left Outer Join on null Field - sql

Question

I have this query
SELECT *
FROM (SELECT mi.visit_id, mi.event_id, mi.patient_id, mi.mrn, mi.reg_date,
mi.d_date, mi.bml_count, mi.TYPE, mblp.baby_patient_id,
mblp.baby_birthdate
FROM ajmid.km0076_motherinfo_test mi LEFT JOIN alfayezb2.mbl_patients mblp
ON mblp.mother_patient_id = mi.patient_id
--works here
AND ( TO_CHAR (mblp.baby_birthdate, 'mm/dd/YYYY') =
TO_CHAR (mi.reg_date, 'mm/dd/YYYY')
OR TO_CHAR (mblp.baby_birthdate, 'mm/dd/YYYY') =
TO_CHAR (mi.reg_date - 1, 'mm/dd/YYYY')
OR TO_CHAR (mblp.baby_birthdate, 'mm/dd/YYYY') =
TO_CHAR (mi.reg_date + 1, 'mm/dd/YYYY')
)
) bml
LEFT OUTER JOIN --doesn't work here
(SELECT ROW_NUMBER () OVER (PARTITION BY vis.patient_id ORDER BY vis.admission_date_time)
num,
vis.admission_date_time, vis.visit_id, vis.patient_id,
vis.facility_id
FROM visit vis) v ON bml.baby_patient_id = v.patient_id
WHERE v.num = 1
ORDER BY bml.reg_date
bml by itself returns 118 rows while the whole query returns 117, the reason is bml returns 1 row with baby_patient_id as null, so I used left outer join to show it, but it's still not showing !!
what can I do to show all rows of bml ?
I'm using Toad 9.6
Thank you

Answer 1

check the query:
SELECT ROW_NUMBER () OVER (PARTITION BY vis.patient_id ORDER BY vis.admission_date_time)
num,
vis.admission_date_time, vis.visit_id, vis.patient_id,
vis.facility_id
FROM visit vis
does it return 118 not null patient_id's?
if it returns 117, that might be the reason.(LEFT OUTER JOIN doesnot pick the records which are null on both tables)
Also, are you sure the null value of baby_patient_id in bml table is actually a NULL value and not a empty charater?(' ').

Answer 2

The probable cause is your filter / criteria (where clause) is eliminating a row with a null value for v.num. The WHERE filters the results after the join.
WHERE v.num = 1 -- Are all v.num equal to 1 ?
The mere action of using a criteria against a field, by definition of what NULL means, eliminates that row from consideration because NULL cannot be evaluated. You can say "WHERE id != 1" and expect to get rows where id is null because null != 1 right? Wrong. id != NULL is not defined logically. It is why we say "IS or IS NOT NULL" when dealing with NULL.

Answer 3

it's working finally !
I added
OR bml.baby_patient_id IS NULL
to the where clause, so the final script is
SELECT *
FROM (SELECT mi.visit_id, mi.event_id, mi.patient_id, mi.mrn, mi.reg_date,
mi.d_date, mi.bml_count, mi.TYPE, mblp.baby_patient_id,
mblp.baby_birthdate
FROM ajmid.km0076_motherinfo_test mi LEFT JOIN alfayezb2.mbl_patients mblp
ON mblp.mother_patient_id = mi.patient_id
AND ( TO_CHAR (mblp.baby_birthdate, 'mm/dd/YYYY') =
TO_CHAR (mi.reg_date, 'mm/dd/YYYY')
OR TO_CHAR (mblp.baby_birthdate, 'mm/dd/YYYY') =
TO_CHAR (mi.reg_date - 1, 'mm/dd/YYYY')
OR TO_CHAR (mblp.baby_birthdate, 'mm/dd/YYYY') =
TO_CHAR (mi.reg_date + 1, 'mm/dd/YYYY')
)
) bml
LEFT OUTER JOIN
(SELECT ROW_NUMBER () OVER (PARTITION BY vis.patient_id ORDER BY vis.admission_date_time)
num,
vis.admission_date_time, vis.visit_id, vis.patient_id,
vis.facility_id
FROM visit vis) v ON bml.baby_patient_id = v.patient_id
WHERE v.num = 1
OR bml.baby_patient_id IS NULL
ORDER BY bml.reg_date
I don't know how this was helpful, I wish someone would explain for me !
Thanks all