SQL joins are used to combine data from two or more database tables based on a related column between them. Joins allow you to retrieve data that spans multiple tables by specifying the relationships between those tables. There are different types of SQL joins, including:
- Inner Join: Returns only the rows that have matching values in both tables being joined. The result set contains only the common records between the tables.
- Left Join (or Left Outer Join): Returns all the rows from the left table and the matched rows from the right table. If there are no matching rows in the right table, NULL values are returned for the right table columns.
- Right Join (or Right Outer Join): Returns all the rows from the right table and the matched rows from the left table. If there are no matching rows in the left table, NULL values are returned for the left table columns.
- Full Join (or Full Outer Join): Returns all the rows from both tables, regardless of whether there is a match or not. If there is no match, NULL values are returned for the columns of the table that lacks a match.
- Cross Join (or Cartesian Join): Produces the Cartesian product of the two tables, meaning it returns all possible combinations of rows from both tables. It doesn’t require a matching column.
To perform a join, you typically use the
JOIN keyword in your SQL query, along with the
ON keyword to specify the join condition. Here’s a simple example of an inner join:
SELECT Orders.OrderID, Customers.CustomerName FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID;
In this example, the
Orders table is joined with the
Customers table on the
CustomerID column, and the query retrieves the
CustomerName columns from the joined result set.
Remember, joins allow you to combine data from multiple tables based on their relationships, enabling you to perform complex queries and retrieve meaningful information from your database.