How can duplicate entries be removed from a PostgreSQL JOIN table?

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To remove duplicate entries from a PostgreSQL JOIN table, you can use the DISTINCT keyword in the SELECT statement to eliminate identical rows, or you can use the GROUP BY clause to group the table by a specific column and remove the duplicates.

For example, suppose you have a JOIN table named "orders" that contains duplicate entries:

SELECT * FROM orders;

id  | customer_id | product_id | quantity
----+-------------+------------+---------
1   | 101         | 201        | 2
2   | 102         | 202        | 3
3   | 101         | 201        | 1
4   | 103         | 203        | 4

To remove the duplicates, you can use the following query:

SELECT DISTINCT * FROM orders;

This query will return:

id  | customer_id | product_id | quantity
----+-------------+------------+---------
1   | 101         | 201        | 2
2   | 102         | 202        | 3
3   | 101         | 201        | 1
4   | 103         | 203        | 4

Alternatively, you can use the GROUP BY clause to group the table by the customerid and productid columns and sum the quantity:

SELECT customer_id, product_id, SUM(quantity) FROM orders GROUP BY customer_id, product_id;

This query will return:

customer_id | product_id | sum
------------+------------+-----
101         | 201        | 3
102         | 202        | 3
103         | 203        | 4

In this case, the duplicates have been removed, and the total quantity for each customer and product is shown.