In SQL, what are joins, and what are the characteristic features of different join types such as inner join, left outer join, right outer join, and full outer join?

Introduction / Context:
Joins are at the heart of relational database querying. Real world data is often normalized into multiple related tables, and joins are the mechanism that allow queries to pull related information together. This question asks what joins are and what distinguishes common join types such as inner join, left outer join, right outer join, full outer join, and self join.

Given Data / Assumptions:

• We have two or more tables that share related columns, such as customer and order tables.
• The database follows relational principles and supports standard SQL join syntax.
• We need to retrieve combined information across tables while controlling which rows appear when there is no match.
• The candidate is expected to know inner and outer join behaviour.

Concept / Approach:
A join is an operation that combines columns from two tables based on a join condition, usually expressed with ON or USING. An inner join returns only rows where the join condition is true on both tables. A left outer join returns all rows from the left table and matches from the right table, filling unmatched columns with nulls. A right outer join is the mirror image. A full outer join returns all rows from both tables, matching where possible and using nulls where no partner exists. A self join joins a table to itself, often using aliases, to compare rows in the same table.

Step-by-Step Solution:
Step 1: Define an inner join such as SELECT * FROM CUSTOMER c INNER JOIN ORDERS o ON c.ID = o.CUSTOMER_ID; which returns only customers that have at least one order.Step 2: Define a left outer join such as SELECT * FROM CUSTOMER c LEFT JOIN ORDERS o ON c.ID = o.CUSTOMER_ID; which returns all customers and any matching orders, with nulls where no order exists.Step 3: Define a right outer join as the opposite perspective, returning all orders and matching customers, depending on which table is on the right side.Step 4: Define a full outer join, where supported, to return all customers and all orders, pairing them where the join condition matches and using nulls otherwise.Step 5: Recognise that a self join uses the same table name twice with different aliases, for example joining employees to managers stored in one EMPLOYEE table.

Verification / Alternative check:
To verify understanding, you can run the same query with different join types and count the rows returned. Inner joins typically return fewer rows because they require matches. Left joins and right joins return at least as many rows as the preserved side table. Full joins usually return the most rows because they include all rows from both tables regardless of matching.

Why Other Options Are Wrong:
Option B confuses joins with indexes, which are separate features. Option C incorrectly claims that joins cannot be used in SELECT, even though SELECT queries are the most common place they appear. Option D suggests that joins permanently merge tables, which is not true; joins operate at query time and do not change physical storage.

Common Pitfalls:
Common mistakes include forgetting the join condition and accidentally creating a Cartesian product, or using the wrong join type and either losing required rows or adding unexpected nulls. Beginners also sometimes confuse the logical meaning of left and right joins when the table order is changed. Understanding the preserved side of an outer join is crucial for writing correct queries.

Final Answer:
Joins combine rows from two or more tables based on a related column, with inner joins returning only matching rows, outer joins returning matching rows plus non matching rows from one or both sides, and self joins joining a table to itself.