Welcome to SQL joins! Joins are fundamental operations that allow us to combine data from multiple tables. In this example, we have a Customers table with customer information and an Orders table with order details. The key is that both tables share a common column - the customer ID - which allows us to link related data together.
INNER JOIN is the most commonly used join type. It returns only the rows that have matching values in both tables. In our example, we join Customers and Orders tables on the customer ID. Notice that Carol from the Customers table doesn't appear in the result because she has no orders. Only Alice and Bob, who have matching records in both tables, appear in the final result.
LEFT JOIN returns all rows from the left table, regardless of whether there's a match in the right table. In our example, all customers appear in the result. Alice and Bob have matching orders, so their order IDs are shown. However, Carol has no orders, so her Order ID shows NULL. This is useful when you want to see all records from one table, even if some don't have corresponding data in the other table.
Let's explore the remaining join types. RIGHT JOIN returns all rows from the right table and matching rows from the left table - it's the opposite of LEFT JOIN. FULL JOIN combines both tables completely, showing all rows from both sides with NULL values where matches don't exist. Finally, CROSS JOIN creates a Cartesian product, combining every row from the first table with every row from the second table, resulting in all possible combinations.
To summarize what we've learned about SQL joins: Joins are essential for combining data from multiple tables using related columns. INNER JOIN returns only matching records, LEFT JOIN keeps all records from the left table, while RIGHT and FULL JOINs provide different coverage options. Choose the appropriate join type based on whether you need all records from one table, both tables, or only matching records.