There are four main types of joins in SQL:
- INNER JOIN returns all rows from both tables where there is a match in the join column(s). This is the most common type of join.
- LEFT JOIN returns all rows from the left table, and the matching rows from the right table. If there is no match in the right table, the corresponding row in the left table will still be returned.
- RIGHT JOIN returns all rows from the right table, and the matching rows from the left table. If there is no match in the left table, the corresponding row in the right table will still be returned.
- FULL OUTER JOIN returns all rows from both tables, whether or not there is a match in the join column(s). This is the most complex type of join.
In addition to these four main types of joins, there are also a few other types of joins, such as:
- CROSS JOIN returns the Cartesian product of two tables, which is the set of all possible combinations of rows from the two tables.
- NATURAL JOIN is a special type of inner join that is used when the join columns have the same name and data type in both tables.
The type of join that you use will depend on the specific requirements of your query. For example, if you want to find all customers who have placed orders, you would use an inner join. If you want to find all customers, even if they have not placed any orders, you would use a left join.
Here is an example of an inner join:
SELECT customer_name, order_id
FROM customers
INNER JOIN orders
ON customers.customer_id = orders.customer_id;
This query will return all rows from the customers table where there is a matching row in the orders table. The customer_name and order_id columns will be returned from both tables.
Here is an example of a left join:
SELECT customer_name, order_id
FROM customers
LEFT JOIN orders
ON customers.customer_id = orders.customer_id;
This query will return all rows from the customers table, whether or not there is a matching row in the orders table. The customer_name column will be returned from the customers table, and the order_id column will be returned from the orders table if there is a match, or NULL if there is no match.
Here is an example of a FULL OUTER JOIN:
SELECT customers.customer_id, customers.name, orders.order_id, orders.order_date
FROM customers
FULL OUTER JOIN orders
ON customers.customer_id = orders.customer_id;
This query will return all rows from the customers table, and all rows from the orders table, whether or not there is a match in the customer_id column. The customer_id, name, order_id, and order_date columns will be returned from both tables.
For example, if there is a customer in the customers table who has not placed any orders, the customer’s row will still be returned in the result set, even though there is no matching row in the orders table. Similarly, if there is an order in the orders table that does not have a matching customer in the customers table, the order’s row will still be returned in the result set, with NULL values for the customer_id and name columns.
The FULL OUTER JOIN is the most complex type of join, but it is also the most versatile. It can be used to find all rows from both tables, even if there are no matches between the tables. This can be useful for finding all customers, even if they have not placed any orders, or for finding all orders, even if they do not have a matching customer.