Using CASE in WHERE clause in Oracle 12c - oracle

Question

I have a select statement like below :
SELECT * FROM
(SELECT d.value, d.line, d.language, l.pos_id, l.vehicle_id,l.vehicle_given
FROM Vehicle_value d,
vehicle_document l
WHERE d.gen_id = l.gen_id
AND d.pos_id = l.pos_id
AND d.gen_id = 1255) d1
LEFT OUTER JOIN
(SELECT d.value, d.line, d.language, l.pos_id, l.vehicle_id,l.vehicle_given
FROM TBB_GEN_Daten d,
TBB_GEN_Lesehilfe l
FROM Vehicle_value d,
vehicle_document l,
WHERE d.gen_id = l.gen_id
AND d.pos_id = l.pos_id
AND d.gen_id = 1254) d2
ON d1.value = d2.value
AND d1.pos_id = d2.pos_id
AND case when d1.vehicle_given = 'YES' then (d1.vehicle_id = d2.vehicle_id)
else end
AND d1.value is not null;
So I wanted to compare the d1.vehicle_id to d2.vehicle_id only when the d1.vehicle_given value is 'YES'. But the case statement i have used here is wrong and hence it doesnt work. is there any other to do this comparision ?

Answer 1

Bunch of typos in your code, but - when fixed, and with CASE being rewritten - query looks like this:
SELECT *
FROM (SELECT d.VALUE,
d.line,
d.language,
l.pos_id,
l.vehicle_id,
l.vehicle_given
FROM Vehicle_value d, vehicle_document l
WHERE d.gen_id = l.gen_id
AND d.pos_id = l.pos_id
AND d.gen_id = 1255) d1
LEFT OUTER JOIN (SELECT d.VALUE,
d.line,
d.language,
l.pos_id,
l.vehicle_id,
l.vehicle_given
FROM TBB_GEN_Daten d,
TBB_GEN_Lesehilfe l,
Vehicle_value d,
vehicle_document l
WHERE d.gen_id = l.gen_id
AND d.pos_id = l.pos_id
AND d.gen_id = 1254) d2
ON d1.VALUE = d2.VALUE
AND d1.pos_id = d2.pos_id
AND d1.vehicle_id =
CASE
WHEN d1.vehicle_given = 'YES' THEN d2.vehicle_id
ELSE d1.vehicle_id
END
AND d1.VALUE IS NOT NULL;

Answer 2

Use brackets and an OR condition for either the matching condition or when it is not YES:
AND ( d1.vehicle_given <> 'YES'
OR d1.vehicle_given IS NULL
OR d1.vehicle_id = d2.vehicle_id )
If you use a CASE expression such as:
AND d1.vehicle_id = CASE
WHEN d1.vehicle_given = 'YES'
THEN d2.vehicle_id
ELSE d1.vehicle_id
END
Then when the ELSE condition is matched you will effectively end up with AND d1.vehicle_id = d1.vehicle_id. This, naively, looks like it should be fine but if d1.vehicle_id is NULL then you will end up comparing NULL = NULL and the result is not truthy and the row will be excluded and that is not what you want.