SQL Joins

Understanding Joins

SQL joins are used to combine rows from two or more tables based on a related column. There are different types of joins, each with a specific purpose:

• INNER JOIN: Returns rows only when there is a match in both tables.
• LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table and matching rows from the right table. If there's no match, it returns NULL for the right table's columns.
• RIGHT JOIN (or RIGHT OUTER JOIN): Returns all rows from the right table and matching rows from the left table. If there's no match, it returns NULL for the left table's columns.
• FULL JOIN (or FULL OUTER JOIN): Returns all rows from both tables. If there's no match, it returns NULL for the columns from the table without a match.
• CROSS JOIN: Returns the Cartesian product of the tables, meaning every possible combination of rows from both tables.

Examples

Here are some examples of how to use joins:

            -- Example: Combining Customers and Orders
            SELECT
                c.CustomerID,
                c.CustomerName,
                o.OrderID,
                o.OrderDate
            FROM
                Customers c
            INNER JOIN
                Orders o ON c.CustomerID = o.CustomerID;
            

            -- Example: Using a LEFT JOIN to get all customers and their orders
            SELECT
                c.CustomerID,
                c.CustomerName,
                o.OrderID,
                o.OrderDate
            FROM
                Customers c
            LEFT JOIN
                Orders o ON c.CustomerID = o.CustomerID;
            

Related Resources

• SQL Joins