Need SQL assistance, Column A and B are distinct, but column c has multiple values. Need to add column D for second distinct value when present

Question

I need help isolating multiple entries in order.

Column A	Column B	Column C
100a	1000	ABC
100a	1000	DEF
200b	2000	GHI
300c	3000	JKL
300c	3000	MNO

There are duplicates in Column A (which has a 1:1 relationship to column b). However, they have a 1:many with column C. Column C will always be unique.

I need to create a report that returns:

Column A	Column B	Column C	Column D
100	1000	ABC	DEF
200	2000	GHI
300	3000	JKL	MNO

Any help? I am pretty new to SQL so this is giving me a lot of trouble figuring it out.

I am at a loss where to start. I tried to declare a variable, but not sure where to start with it.

Answer 1

If it's no more than 2 values in Column C for Column A + Column B key, then a query below would work. It will not work for 3 or more values

SELECT 
  Column_A, 
  Column_B, 
  MIN(Column_C) AS "Column C", 
  CASE WHEN MIN(Column_C) <> MAX(Column_C) 
     THEN MAX(Column_C)
     ELSE ''
  END AS "Column D"
FROM some_table
GROUP BY Column_A, Column_B

Column_A	Column_B	Column C	Column D
100a	1000	ABC	DEF
200b	2000	GHI
300c	3000	JKL	MNO

(live demo: dbfiddle )