Here are 50 commonly asked SQL interview questions for both interviews and written exams:

Basic SQL Questions:

  1. What is SQL, and what does it stand for?
  2. Explain the differences between SQL and NoSQL databases.
  3. What are the key components of an SQL statement?
  4. What is a database, and what is the role of a database management system (DBMS)?

SELECT Statements:

  1. How do you retrieve all records from a table in SQL?
  2. What is the SQL statement to select specific columns from a table?
  3. How can you filter records using the WHERE clause in SQL?
  4. Explain the difference between the DISTINCT and GROUP BY clauses.
  5. How do you limit the number of records returned by a query?

Table Operations:

  1. How do you create a new table in SQL?
  2. What is the purpose of the PRIMARY KEY constraint?
  3. Explain the differences between the INSERT, UPDATE, and DELETE statements.
  4. How can you add a new column to an existing table?
  5. What is the purpose of the FOREIGN KEY constraint, and how is it used?

Joins and Relationships:

  1. What is an SQL JOIN, and what are the different types of JOINs?
  2. How do you retrieve data from multiple tables using a JOIN?
  3. What is a self-join, and when would you use it?
  4. Explain the concept of a one-to-many relationship in database design.
  5. How do you find records with no matching values in another table?

Aggregation and Grouping:

  1. What are aggregate functions in SQL, and provide examples of a few.
  2. How do you calculate the total number of records in a table?
  3. Explain the HAVING clause and its purpose.
  4. How do you find the average, sum, and maximum values of a column?
  5. What is the difference between the COUNT(*) and COUNT(column_name) functions?

Subqueries and Nesting:

  1. What is a subquery, and how is it different from a JOIN?
  2. How do you use a subquery to filter results in the WHERE clause?
  3. Provide an example of a correlated subquery.
  4. Explain the EXISTS and NOT EXISTS operators in subqueries.
  5. How do you use the IN and NOT IN operators with subqueries?

Indexes and Optimization:

  1. What is an index in a database, and why is it important?
  2. How can you improve query performance using indexes?
  3. Explain the concept of query optimization in SQL.
  4. What is the purpose of the EXPLAIN statement in SQL?

Transactions and ACID:

  1. What is a database transaction, and what does ACID stand for?
  2. How do you start and commit a transaction in SQL?
  3. Explain the concept of isolation levels in database transactions.
  4. What is a deadlock, and how can it be resolved in SQL?

Views and Stored Procedures:

  1. What is an SQL view, and how is it different from a table?
  2. How do you create a stored procedure in SQL?
  3. What are the advantages of using views and stored procedures?

Security and Permissions:

  1. How do you grant and revoke permissions on database objects?
  2. What is SQL injection, and how can it be prevented?
  3. Explain the concept of role-based access control in SQL.

Normalization and Database Design:

  1. What is database normalization, and why is it important?
  2. Describe the different normal forms in database design.
  3. How do you denormalize a database, and when is it appropriate to do so?

Backup and Recovery:

  1. How do you create a backup of a database in SQL?
  2. Explain the process of restoring a database from a backup.

Error Handling:

  1. What is the purpose of the TRY…CATCH block in SQL for error handling?
  2. How do you raise custom errors in SQL?

These SQL interview questions cover a wide range of topics and are often used to assess a candidate’s knowledge and skills in working with relational databases

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