SQLite 3.27

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CREATE TABLE test_table (
  `customer` VARCHAR(5),
  `product` VARCHAR(6),
  `quantity` INTEGER,
  `date_of_purchase` TEXT
);
✓
INSERT INTO test_table
  (`customer`, `product`, `quantity`, `date_of_purchase`)
VALUES
  ('John', 'coffee', '1', '02.20.2019'),
  ('Bob', 'coffee', '2', '01.13.2018'),
  ('Chris', 'milk', '1', '02.03.2012'),
  ('Alice', 'bread', '1', '05.01.2019'),
  ('John', 'banana', '2', '08.04.2015'),
  ('Mary', 'orange', '3', '12.16.2018'),
  ('Mary', 'orange', '2', '04.21.2019'),
  ('Chris', 'pie', '1', '07.02.2004');
✓
SELECT t.*
FROM test_table t
WHERE EXISTS (SELECT 1 FROM test_table WHERE customer = t.customer AND product <> t.product)


customer
product
quantity
date_of_purchase


John
coffee
1
02.20.2019

Chris
milk
1
02.03.2012

John
banana
2
08.04.2015

Chris
pie
1
07.02.2004


SELECT customer, 
       GROUP_CONCAT(product) products,
       GROUP_CONCAT(date_of_purchase) dates
FROM test_table
GROUP BY customer
HAVING COUNT(DISTINCT product) > 1


customer
products
dates


Chris
milk,pie
02.03.2012,07.02.2004

John
coffee,banana
02.20.2019,08.04.2015

customer	product	quantity	date_of_purchase
John	coffee	1	02.20.2019
Chris	milk	1	02.03.2012
John	banana	2	08.04.2015
Chris	pie	1	07.02.2004

customer	products	dates
Chris	milk,pie	02.03.2012,07.02.2004
John	coffee,banana	02.20.2019,08.04.2015