Select all rows with values that appear twice by the same customer? - mysql

Question

I have a table:
CREATE TABLE Orders (
ID INT,
Customer INT,
PRIMARY KEY(ID)
);
CREATE TABLE Items (
ID INT,
Barcode INT,
PRIMARY KEY(ID, Barcode)
);
INSERT INTO Orders VALUES
(1, 1), (2, 1), (3, 2), (4, 3), (5, 3);
INSERT INTO Items VALUES
(1, 1), (1, 2), (1, 3), (1, 7),
(2, 1), (2, 3), (3, 2), (3, 8),
(4, 2), (4, 3), (4, 8), (5, 4);
I'm trying to find all customers who have ordered the same item twice and specify the item, but not from the same order. I just need a list of Orders.Customer and Items.Barcode showing this.
Here's a query that helps illustrate:
SELECT i.ID, i.Barcode, o.Customer
FROM Items i, Orders o
WHERE i.ID = o.ID
Which produces the below:
+----+---------+----------+
| ID | Barcode | Customer |
+----+---------+----------+
| 1 | 1 | 1 | # A
| 1 | 2 | 1 |
| 1 | 3 | 1 | # B
| 1 | 7 | 1 |
| 2 | 1 | 1 | # A
| 2 | 3 | 1 | # B
| 3 | 2 | 2 |
| 3 | 8 | 2 |
| 4 | 2 | 3 |
| 4 | 3 | 3 |
| 4 | 8 | 3 |
| 5 | 4 | 3 |
+----+---------+----------+
Note where I tagged A, Barcode 1 appears in both ID 1 and ID 2. Both those orders have the same customer, same barcode, but different order IDs. B is another example.
How can I pull out these rows, so I have something like the below:
+---------+----------+
| Barcode | Customer |
+---------+----------+
| 1 | 1 |
| 3 | 1 |
+---------+----------+
More declaratively, I want to know what customers have ordered the same item twice, and list the items and customers. In other words, "Customer 1 has ordered Items 1 and 3 twice".

Answer 1

I'm trying to find all customers who have ordered the same item twice and specify the item, but not from the same order.
This is pretty simple with a HAVING clause:
SELECT o.Customer, i.Barcode
FROM Orders o JOIN
Items i
ON i.ID = o.ID
GROUP BY o.Customer, i.Barcode
HAVING MIN(o.id) <> MAX(o.id);
Note the use of proper, explicit, standard, readable JOIN syntax. Never use commas in the FROM clause.

Answer 2

If you need the expected result then you should group by with having clause like below.
SELECT i.Barcode, o.Customer
FROM Items i, Orders o
WHERE i.ID = o.ID
GROUP BY i.Barcode, o.Customer HAVING COUNT(*) >1
I am assuming that you need all records which are repeating more than once.

Answer 3

You may try this -
With cte1 as (SELECT i.ID as orderId, i.Barcode, o.Customer
FROM Items i, Orders o
WHERE i.ID = o.ID),
cte2 as (SELECT i.ID as orderId, i.Barcode, o.Customer
FROM Items i, Orders o
WHERE i.ID = o.ID)
Select distinct cte1.Barcode, cte1.Customer
from cte1, cte2
where
cte1.Barcode = cte2.Barcode
and cte1.Customer = cte2.Customer
and cte1.orderId <> cte2.orderId;
More elegant way - Please refer Amit Verma's answer