Question 1

Explain the different types of SQL JOINs.

Accepted Answer

INNER JOIN returns matching rows from both tables. LEFT JOIN returns all rows from the left table + matches from the right (NULL if no match). RIGHT JOIN is the reverse. FULL OUTER JOIN returns all rows from both tables.

Question 2

What is database indexing? When should you add an index?

Accepted Answer

An index is a data structure (typically B-Tree) that allows the database to find rows without scanning the entire table. Add indexes on columns used in WHERE, JOIN ON, and ORDER BY clauses — but too many indexes slow down writes.

Question 3

What are ACID properties in database transactions?

Accepted Answer

ACID: Atomicity (all or nothing), Consistency (valid state transitions), Isolation (concurrent transactions don't interfere), Durability (committed data survives crashes). These properties guarantee reliable transactions.