What is a join? Name different types.

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What is a JOIN?

A JOIN in SQL is used to combine rows from two or more tables based on a related column (usually a primary key in one table and a foreign key in another).

It allows us to retrieve meaningful data that is spread across multiple tables.

Types of Joins in SQL

  1. INNER JOIN

    • Returns only the rows where there is a match in both tables.

    • Example: Students enrolled in a course.

    SELECT s.name, c.course_name
FROM Students s
INNER JOIN Courses c ON s.course_id = c.course_id;

  1. LEFT JOIN (LEFT OUTER JOIN)

    • Returns all rows from the left table and matching rows from the right table.

    • If no match exists, NULL is returned for right table columns.

  1. RIGHT JOIN (RIGHT OUTER JOIN)

    • Returns all rows from the right table and matching rows from the left table.

    • If no match exists, NULL is returned for left table columns.

  1. FULL JOIN (FULL OUTER JOIN)

    • Returns all rows from both tables.

    • If there’s no match, NULLs are returned for missing data.

  1. CROSS JOIN

    • Returns the Cartesian product of two tables (every row from the first table combined with every row from the second).

    • Can produce very large results.

  1. SELF JOIN

    • A table joined with itself.

    • Useful for hierarchical or relational data within the same table (e.g., employees and their managers).

✅ In short:

  • INNER JOIN → Matching data only

  • LEFT JOIN → All from left + matches

  • RIGHT JOIN → All from right + matches

  • FULL JOIN → All from both

  • CROSS JOIN → Every combination

  • SELF JOIN → Table with itself

Read More:

What is normalization?

What are primary keys and foreign keys?

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