What is a join, and what are the different types of joins?

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What is a join, and what are the different types of joins?

Alejandro Penzini Answered question October 30, 2023
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A join in SQL is a clause that combines rows from two or more tables based on a common field between them. Joins are used to create more complex queries that retrieve data from multiple tables.

There are four main types of joins in SQL:

Inner join: An inner join returns all rows from both tables where the common field matches.
Left join: A left join returns all rows from the left table, even if there is no matching row in the right table.
Right join: A right join returns all rows from the right table, even if there is no matching row in the left table.
Full outer join: A full outer join returns all rows from both tables, even if there is no matching row in the other table.
Here is an example of an inner join:

SQL
SELECT *
FROM customers
INNER JOIN orders ON customers.id = orders.customer_id;
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This query will return all rows from the customers and orders tables where the id column in the customers table matches the customer_id column in the orders table.

Here is an example of a left join:

SQL
SELECT *
FROM customers
LEFT JOIN orders ON customers.id = orders.customer_id;
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This query will return all rows from the customers table, even if there is no matching row in the orders table.

Here is an example of a right join:

SQL
SELECT *
FROM customers
RIGHT JOIN orders ON customers.id = orders.customer_id;
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This query will return all rows from the orders table, even if there is no matching row in the customers table.

Here is an example of a full outer join:

SQL
SELECT *
FROM customers
FULL JOIN orders ON customers.id = orders.customer_id;
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This query will return all rows from both the customers and orders tables, even if there is no matching row in the other table.

Joins are a powerful tool for retrieving data from multiple tables in SQL. By understanding the different types of joins, you can write more complex and efficient queries.

Alejandro Penzini Answered question October 30, 2023
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