SQL joins are operations that allow developers to combine records from two or more tables in a relational database based on a related column between them. By using joins, you can retrieve complex data sets that reflect real-world relationships without duplicating data in your database. The most common types of joins include INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN. Each of these types serves a different purpose in how data is matched and displayed from the involved tables.
For instance, an INNER JOIN returns rows when there are matching values in both tables. If you have a Customers table and an Orders table, an INNER JOIN on the customer ID would only show customers who have placed orders. In contrast, a LEFT JOIN would include all customers, showing their orders if they exist, or returning NULLs where there are no orders. This is useful when you want a comprehensive list of customers regardless of whether they have made a purchase.
Using joins enhances the efficiency of queries by allowing developers to retrieve related data in a single query rather than pulling data from multiple tables individually. This not only simplifies data retrieval but also maintains data integrity and reduces redundancy. For example, instead of having duplicate customer information in every order record, you can store customer details once in the Customers table and reference that data through joins, making your database clearer and easier to manage.