SQL: case when statement with over (partition by) - sql

Question

I'm quite new here and have tried various hints from Stackoverflow posts on SQL but haven't been able to solve this one.
I have a table that is result of tables joined, looks like this
Table A
cust_id prod_type
001 A
001 A
002 A
002 B
003 A
003 C
I need to apply logic: If for each cust_id there is at least one value where prod_type is B or C, then return corresponding prod_type value. If for each cust_id all values of prod_type are A, return A.
The final output i am trying to get is
Table B
cust_id prod_type
001 A
002 B
003 C
I have tried using
SELECT
A.cust_id
,CASE WHEN prod_type in ('B', 'C') THEN prod_type OVER (PARTITION BY A.cust_id)
ELSE 'A' OVER (PARTITION BY A.cust_id) END AS product
FROM ([Joined Tables]) AS A
and it seems that teradata does not allow to use over(clause) in a case statement: expects 'END' keyword between prod_type and OVER keyword.

Answer 1

You want to return only one row per customer with the best matching product_type?
If there are additional columns:
SELECT
A.cust_id
,prod_type
,...
FROM ([Joined Tables]) AS A
QUALIFY
ROW_NUMBER()
OVER (PARTITION BY CUST_ID
ORDER BY CASE WHEN prod_type in ('B', 'C') -- best match first
THEN 1
ELSE 2
END,
prod_type) = 1
Otherwise #Frisbee's MAX will work, but I assume that A/B/C are not your actual product names:
SELECT
A.cust_id
,COALESCE(MAX(CASE WHEN prod_type in ('B', 'C') THEN prod_type END)
,MAX(CASE WHEN prod_type not in ('B', 'C') THEN prod_type END))
FROM ([Joined Tables]) AS A
GROUP BY just_id

Answer 2

For completness here is an equivalent using the function first_value and using the inverse alphabetical order of your products.
Similarly to #Amgalan Bilegjav answer, 'b' is the sample table and 'a' the table with an extra column (finding the first product here).
The code was tested with Teradata version 16.20.53.55
SELECT
a.CUST_ID
, a.PROD_TYPE
FROM (
SELECT
b.CUST_ID
, first_value(b.PROD_TYPE) over (partition by b.CUST_ID order by b.PROD_TYPE desc) as PROD_TYPE
FROM (
/* recreate the exemple table */
select * from (SELECT '001' CUST_ID, 'A' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '001' CUST_ID, 'A' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '002' CUST_ID, 'A' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '002' CUST_ID, 'B' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '003' CUST_ID, 'A' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '003' CUST_ID, 'C' PROD_TYPE) as "DUAL"
) b
) a
group by a.CUST_ID, a.PROD_TYPE
order by a.CUST_ID
;

Answer 3

This query recreates your sample data (table b.) Add the row numbers based on specific product order (ranging from C, B to A in the table a) then filter on the first row to get the wanted table.
SELECT
a.CUST_ID
, a.PROD_TYPE
FROM (
SELECT
b.CUST_ID
, b.PROD_TYPE
, ROW_NUMBER() OVER (PARTITION BY CUST_ID ORDER BY DECODE(PROD_TYPE, 'A',3,'B',2,'C',1)) as RN
FROM (
/* recreate the exemple table */
select * from (SELECT '001' CUST_ID, 'A' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '001' CUST_ID, 'A' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '002' CUST_ID, 'A' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '002' CUST_ID, 'B' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '003' CUST_ID, 'A' PROD_TYPE) as "DUAL" UNION all
select * from (SELECT '003' CUST_ID, 'C' PROD_TYPE) as "DUAL"
) b
) a
WHERE RN = 1
order by a.CUST_ID
;