Mastering SQL: Essential Interview Questions and Answers

SQL is a cornerstone of database management and a must-have skill for many tech roles. Whether you're starting your career or brushing up for an advanced position, this guide covers the most important SQL interview questions and answers. Let's get you prepared to ace your next interview!

1. What is SQL?

SQL, or Structured Query Language, is a standard programming language used to manage and manipulate relational databases. It enables operations such as creating, reading, updating, and deleting data (commonly referred to as CRUD operations).

2. What are the key types of SQL commands?

• DDL (Data Definition Language): Defines the structure of the database using commands like CREATE, ALTER, DROP.
• DML (Data Manipulation Language): Manages data using SELECT, INSERT, UPDATE, DELETE.
• DCL (Data Control Language): Manages access to data with GRANT and REVOKE.
• TCL (Transaction Control Language): Controls transaction behavior with COMMIT, ROLLBACK, and SAVEPOINT.

3. What is the difference between a primary key and a unique key?

• Primary Key: Uniquely identifies a record in a table and does not allow NULL values. A table can have only one primary key.
• Unique Key: Ensures the uniqueness of a column but allows a single NULL value. A table can have multiple unique keys.

4. Explain the concept of normalization and its types.

Normalization organizes data to minimize redundancy and ensure logical data storage. The main forms are:

• 1NF: Eliminates duplicate columns and ensures atomic data.
• 2NF: Ensures every non-primary attribute is fully functionally dependent on the primary key.
• 3NF: Removes transitive dependencies.
• BCNF: A stricter form of 3NF ensuring every determinant is a candidate key.

5. What are SQL joins?

Joins combine rows from multiple tables. Common types include:

• INNER JOIN: Returns matching records from both tables.
• LEFT JOIN: Returns all records from the left table and matches from the right.
• RIGHT JOIN: Returns all records from the right table and matches from the left.
• FULL OUTER JOIN: Returns all records where there is a match in either table.

6. What is an index, and why is it used?

An index improves data retrieval speed at the cost of additional storage. Types of indexes include:

• Clustered Index: Sorts and stores data rows in the table.
• Non-Clustered Index: Creates a separate structure for quick lookups.
• Unique Index: Prevents duplicate values in a column.

7. What is a stored procedure?

A stored procedure is a precompiled SQL code that performs a specific task. It can accept parameters and return results, enhancing reusability and performance.

    CREATE PROCEDURE GetCustomerDetails
    AS
    SELECT * FROM Customers
    WHERE Country = 'USA';
    

8. What is a trigger?

A trigger is a SQL code automatically executed in response to certain events on a table, like INSERT, UPDATE, or DELETE. Triggers ensure consistency and enforce business rules.

9. Explain ACID properties in SQL.

ACID ensures reliable database transactions:

• Atomicity: Transactions are all-or-nothing.
• Consistency: Transactions leave the database in a valid state.
• Isolation: Transactions occur independently.
• Durability: Committed changes persist despite failures.

10. Advanced SQL Techniques

Enhance your SQL skills with techniques like:

• Common Table Expressions (CTEs): Simplify complex queries.
• Window Functions: Perform calculations across query rows.
• Dynamic SQL: Build and execute SQL queries at runtime.

Mastering these concepts will prepare you for SQL interviews, helping you confidently tackle any challenge. Keep practicing and stay updated with the latest trends in SQL!

For more SQL resources, visit CodeToCareer Blog.

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