What is the purpose of INNER JOIN in SQL?

The purpose of INNER JOIN in SQL is to combine rows from two or more tables based on a related column between them. Here are the key points:

1. Retrieve Matching Records: INNER JOIN returns only the rows where there is a match in both tables. If there is no match, those rows are excluded from the result set.

2. Combine Data: It allows you to retrieve data from multiple tables in a single query, which is useful for analyzing related data. For example, you can join a customers table with an orders table to get customer details along with their orders.

3. Specify Join Conditions: You define the relationship between the tables using the ON clause, specifying which columns to match.

4. Improve Query Efficiency: By joining tables, you can reduce the amount of data processed and returned, leading to more efficient queries.

Here’s a simple example of an INNER JOIN:

SELECT orders.order_id, customers.first_name
FROM orders
INNER JOIN customers ON orders.customer_id = customers.customer_id;

This query retrieves order IDs along with the first names of customers who placed those orders.