Create a new column in a sql query using case statement

Question

I have done a few table joins and below is how my data looks.

Cust_No Account_No  Product_H_f Product_H_L
123     A9023         Core      Training
123     A9023         Core      Training
834     A9023         Core      Complimentary
634     A9024         Core      Complimentary
264     A9025         DTB       Training
263     A9025         DTB       Complimentary
677     A9025         DTB       Training

I want to add a column to this result that would be called 'Customer_no who have Training' and should look like this. (My original query is pretty complex with joins on 6 tables using left join)

Cust_No Account_No  Product_H_f Product_H_L Cust_has_Training
123    A9023        Core        Training        Yes
123    A9023        Core        Training        Yes
834    A9023        Core        Complimentary   Yes 
834    A9023        Core        Training        Yes
634    A9024        Core        Complimentary   No
264    A9025        DTB         Training        Yes
263    A9025        DTB         Complimentary   No
677    A9025        DTB         Training        Yes
677    A9025        DTB         Basic           Yes

I am using the below code to achieve this and it seems to be working. But since I have 48M records, it is taking forever to run. Is there a way I can optimize this query?

SELECT Cust_No, 
       Account_No,  
       Product_H_f, 
       Product_H_L,
(CASE WHEN Cust_No IN (SELECT distinct(Cust_No) from table where 
Product_H_L='Training') then 'Yes' else 'No' end) as 'Cust_has_Training'
FROM TABLE 

Answer 1

If you got a complex query the most efficient (and easiest) way to add this new calculation is probably based on Windowed Aggregates:

SELECT Cust_No, 
       Account_No,  
       Product_H_f, 
       Product_H_L,
       -- returns YES when any row for a customer has 'Training'
       MAX(CASE WHEN Product_H_L='Training' THEN 'Yes' else 'No' end)
       OVER (PARTITION BY Cust_No) as 'Cust_has_Training'
FROM TABLE