Structured Query Language (SQL) is a powerful tool for managing and manipulating data in relational databases. One of the most important features of SQL is the ability to join tables together to create more complex queries. In this article, we will explore the different types of SQL joins and how they can be used to combine data from multiple tables.
Inner Join
The inner join is the most commonly used type of join in SQL. It returns only the rows that have matching values in both tables being joined. For example, if we have two tables, one containing customer information and another containing order information, we can use an inner join to combine the two tables and get a list of all customers who have placed an order.
SELECT customers.name, orders.order_date
FROM customers
INNER JOIN orders
ON customers.id = orders.customer_id;
In this example, we are selecting the customer’s name and the order date from the two tables. The INNER JOIN clause specifies that we want to join the two tables on the customer ID column in the customers table and the customer ID column in the orders table.
Outer Join
An outer join returns all rows from one table and matching rows from another table. If there are no matching rows in the second table, the result will contain null values for those columns. There are two types of outer joins: left outer join and right outer join.
Left Outer Join
A left outer join returns all rows from the left table and matching rows from the right table. If there are no matching rows in the right table, the result will contain null values for those columns. For example, if we have a table of customers and a table of orders, we can use a left outer join to get a list of all customers and their orders, even if they haven’t placed an order yet.
SELECT customers.name, orders.order_date
FROM customers
LEFT OUTER JOIN orders
ON customers.id = orders.customer_id;
In this example, we are selecting the customer’s name and the order date from the two tables. The LEFT OUTER JOIN clause specifies that we want to join the two tables on the customer ID column in the customers table and the customer ID column in the orders table.
Right Outer Join
A right outer join returns all rows from the right table and matching rows from the left table. If there are no matching rows in the left table, the result will contain null values for those columns. For example, if we have a table of orders and a table of customers, we can use a right outer join to get a list of all orders and their associated customers, even if the customer information is missing.
SELECT customers.name, orders.order_date
FROM customers
RIGHT OUTER JOIN orders
ON customers.id = orders.customer_id;
In this example, we are selecting the customer’s name and the order date from the two tables. The RIGHT OUTER JOIN clause specifies that we want to join the two tables on the customer ID column in the customers table and the customer ID column in the orders table.
Full Outer Join
A full outer join returns all rows from both tables and matching rows from both tables. If there are no matching rows in one of the tables, the result will contain null values for those columns. For example, if we have a table of customers and a table of orders, we can use a full outer join to get a list of all customers and their orders, including those who haven’t placed an order yet and those whose order information is missing.
SELECT customers.name, orders.order_date
FROM customers
FULL OUTER JOIN orders
ON customers.id = orders.customer_id;
In this example, we are selecting the customer’s name and the order date from the two tables. The FULL OUTER JOIN clause specifies that we want to join the two tables on the customer ID column in the customers table and the customer ID column in the orders table.
Conclusion
SQL joins are a powerful tool for combining data from multiple tables. The inner join is the most commonly used type of join, but outer joins can be useful when you need to include all rows from one table, even if there are no matching rows in the other table. Understanding the different types of SQL joins and how they work can help you write more complex queries and get the data you need from your databases.
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- Source: https://zephyrnet.com/understanding-sql-joins-inner-outer-full-left-and-right/