SQL Query to apply join on basis of case Condition - sql

Question

I have a requirement where I need to fetch the Dimension Key of Region table on basis of the following preference.
Fetch dimension key on basis of Zipcode of Physical address(PA)
If the first condition is not satisfied that fetch dimension key on basis of the Zip Code of the Mailing address
If the second condition is also not satisfied than fetch the dimension key on basis of the Parish Code of Physical address
Else fetch dimension key on basis of parish Code of Mailing address.
I am trying to use the below query but is giving multiple records since all left joins are getting evaluated. I want that it should not go on the second condition if the first condition is satisfied.
select REGION_DIM_SK, CASE_NUM
from (
select distinct COALESCE(RDIM.REGION_DIM_SK, RDIM1.REGION_DIM_SK, RDIM2.REGION_DIM_SK, RDIM3.REGION_DIM_SK) AS REGION_DIM_SK
, DC.CASE_NUM, ADDR_TYPE_CD
FROM rpt_dm_ee_intg.CASE_PERSON_ADDRESS dc
left join rpt_dm_ee_prsnt.REGION_DIM RDIM on dc.ZIP_CODE = RDIM.ZIP_CODE and RDIM.REGION_EFF_END_DT IS NULL and dc.addr_type_cd='PA' AND dc.EFF_END_DT IS NULL
left join rpt_dm_ee_prsnt.REGION_DIM RDIM1 ON dc.ZIP_CODE = RDIM1.ZIP_CODE AND RDIM1.REGION_EFF_END_DT IS NULL AND dc.addr_type_cd='MA' AND DC.EFF_END_DT IS NULL
left join (
select PARISH_CD, min(REGION_DIM_SK) as REGION_DIM_SK
from rpt_dm_ee_prsnt.REGION_DIM
where REGION_EFF_END_DT is null
group by PARISH_CD
) RDIM2 ON dc.addr_type_cd='PA' and dc.PARISH_CD = RDIM2.PARISH_CD AND DC.EFF_END_DT IS NULL
left join (
select PARISH_CD, min(REGION_DIM_SK) as REGION_DIM_SK
from rpt_dm_ee_prsnt.REGION_DIM
where REGION_EFF_END_DT is null
group by PARISH_CD
) RDIM3 ON dc.addr_type_cd='MA' and dc.PARISH_CD = RDIM3.PARISH_CD AND DC.EFF_END_DT IS NULL
) A
where REGION_DIM_SK is not null
) RD on RD.case_num = rpt_dm_ee_intg.CASE_PERSON_ELIGIBILITY.CASE_NUM

Answer 1

Use multiple left joins. Your query is rather hard to follow -- it has other tables and references not described in the problem.
But the idea is:
select t.*,
coalesce(rpa.dim_key, rm.dim_key, rpap.dim_key, rmp.dim_key) as dim_key
from t left join
dim_region rpa
on t.physical_address_zipcode = rpa.zipcode left join
dim_region rm
on t.mailing_address_zipcode = rm.zipcode and
rpa.zipcode is null left join
dim_region rpap
on t.physical_addresss_parishcode = rpap.parishcode and
rm.zipcode is null left join
dim_region rmp
on t.physical_addresss_parishcode = rmp.parishcode and
rpap.zipcode is null

Answer 2

The trick is to put the conditions in CASE WHEN:
SELECT *
FROM table1 a
JOIN table2 b
ON CASE
WHEN a.code is not null and a.code = b.code THEN 1
WHEN a.type = b.type THEN 1
ELSE 0
END = 1
For your example you can reduce the code to just two joins, it can't be done in one as you are joining two different tables.
SELECT CASE WHEN RDIM.addres IS NULL THEN RDIM2.addres ELSE RDIM.addres
FROM rpt_dm_ee_intg.CASE_PERSON_ADDRESS dc
LEFT JOIN rpt_dm_ee_prsnt.REGION_DIM RDIM ON CASE
WHEN (dc.ZIP_CODE = RDIM.ZIP_CODE
AND RDIM.REGION_EFF_END_DT IS NULL
AND dc.addr_type_cd='PA'
AND dc.EFF_END_DT IS NULL) THEN 1
WHEN (dc.ZIP_CODE = RDIM1.ZIP_CODE
AND RDIM1.REGION_EFF_END_DT IS NULL
AND dc.addr_type_cd='MA'
AND DC.EFF_END_DT IS NULL) THEN 1
ELSE 0
END = 1
LEFT JOIN
(SELECT PARISH_CD,
min(REGION_DIM_SK) AS REGION_DIM_SK
FROM rpt_dm_ee_prsnt.REGION_DIM
WHERE REGION_EFF_END_DT IS NULL
GROUP BY PARISH_CD) RDIM2 ON CASE
WHEN (dc.addr_type_cd='PA'
AND dc.PARISH_CD = RDIM2.PARISH_CD
AND DC.EFF_END_DT IS NULL
AND RDIM.ZIP_CODE IS NULL) THEN 1
WHEN (dc.addr_type_cd='MA'
AND dc.PARISH_CD = RDIM3.PARISH_CD
AND DC.EFF_END_DT IS NULL
AND RDIM.ZIP_CODE IS NULL) THEN 1
ELSE 0
END = 1
edit
If you don't want to have nulls from RDIM2 table if RDIM1 zip code is present the logic could be easily extended to support that. You just need to add AND RDIM.ZIP_CODE IS NULL to CASE WHEN conditions.