Inner join 允许重复吗？

Question

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

Answer 1

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)

Answer 2

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 |
+----+----------+--------+---------------------+