Will Inner join allow duplicates? - sql

Question

if join two tables using inner join method will it return duplicate values ?

Answer 1

Yes, if there are duplicate values.
If you have CUSTOMERS table:
+----+----------+-----+-----------+----------+
| ID | NAME | AGE | ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+
and ORDERS table as follows:
+-----+---------------------+-------------+--------+
| OID | DATE | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
| 101 | 2009-11-20 00:00:00 | 2 | 1560 |
| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
+-----+---------------------+-------------+--------+
Then inner join will give result:
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
INNER JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
This would produce the following result:
+----+----------+--------+---------------------+
| ID | NAME | AMOUNT | DATE |
+----+----------+--------+---------------------+
| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
+----+----------+--------+---------------------+

Answer 2

Here is an example with duplicate rows in both tables.
select * from customers;
id | name | age | address
----+-------------+-----+-----------
1 | Ramesh | 32 | Ahmedabad
2 | Khilan | 25 | Delhi
3 | kaushik | 23 | Kota <-- id 3 "kaushik"
3 | kaushik_two | 23 | Ahmedabad <-- appears twice
4 | Chaitali | 25 | Mumbai
5 | Hardik | 27 | Bhopal
6 | Komal | 22 | MP
7 | Muffy | 24 | Indore
(8 rows)
select * from orders;
oid | date | customer_id | amount
-----+---------------------+-------------+--------
102 | 2009-10-08 00:00:00 | 3 | 3000 <-- reference to customer 3
100 | 2009-10-08 00:00:00 | 3 | 1500 <-- also appears twice
101 | 2009-11-20 00:00:00 | 2 | 1560
103 | 2008-05-20 00:00:00 | 4 | 2060
104 | 2022-01-01 00:00:00 | 100 | 3900
(5 rows)
Inner Join
Produces duplicated rows for "kaushik" and "kaushik_two".
select id, name, amount, date
from customers
inner join orders on customers.id = orders.customer_id;
id | name | amount | date
----+-------------+--------+---------------------
2 | Khilan | 1560 | 2009-11-20 00:00:00
3 | kaushik | 1500 | 2009-10-08 00:00:00 <-- first pair
3 | kaushik | 3000 | 2009-10-08 00:00:00
3 | kaushik_two | 1500 | 2009-10-08 00:00:00 <-- second pair
3 | kaushik_two | 3000 | 2009-10-08 00:00:00
4 | Chaitali | 2060 | 2008-05-20 00:00:00
(6 rows)