SQLite 3.39

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CREATE TABLE some_table (
  Column_A VARCHAR(50),
  Column_B VARCHAR(50),
  Column_C VARCHAR(50))
✓
INSERT INTO some_table (Column_A, Column_B, Column_C) 
  VALUES 
			('100a', '1000', 'ABC'),
			('100a', '1000', 'DEF'),
			('200b', '2000', 'GHI'),
			('300c', '3000', 'JKL'),
			('300c', '3000', 'MNO')
✓
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