SQL Joins Explained: How to Link Database Tables Without Duplicating Data

SQL joins allow developers to combine data from multiple related tables using shared key columns such as customer_id or product_id. The four main join types — INNER, LEFT, RIGHT, and SELF — each serve a distinct purpose depending on whether unmatched rows need to be included. An INNER JOIN returns only rows with matching values in both tables, while a LEFT JOIN retains all rows from the left table even when no match exists on the right. A SELF JOIN enables a table to be compared against itself, useful for finding rows that share a common attribute. INNER JOIN and LEFT JOIN cover most real-world use cases, making RIGHT JOIN largely redundant in practice.