SQL Interview Questions | PROGRAMING WIZARDS

SQL, a standard database language, is not just a theoretical concept but a practical tool used for accessing and manipulating database data. It was developed by IBM Computer Scientists in the 1970s and has since been a crucial part of database management. By executing queries, SQL can create, update, delete, and retrieve data in databases like MySQL, Oracle, PostgreSQL, etc. Overall, SQL is a query language that communicates with databases, making it an essential skill for any SQL developer or job seeker.

In this article, we cover 20+ SQL interview questions with answers asked in SQL developer interviews at MAANG and other high-paying companies. Whether you are a fresher or an experienced professional with 2, 5, or 10 years of experience, this article gives you all the confidence you need to ace your following SQL interview.

SQL Interview Questions and Answers for Freshers

What is SQL?

SQL stands for Structured Query Language. It is a language used to interact with databases, allowing users to create, update, delete, and retrieve data. For example, if you're a sales manager, you can use SQL to retrieve the total sales for a specific product or update the price of a product. SQL is an ANSI (American National Standards Institute) standard. Using SQL, you can execute queries, insert records, update records, create databases, create tables, and delete tables.

What is a database?

A database is a structured form of data storage in a computer, organized to access, manage, and update data easily. It includes schemas (a collection of database objects), tables (a collection of related data entries that consists of rows and columns), queries (a request for data or information from a database), views (a virtual table based on the result set of an SQL query), and more. A Database Management System (DBMS) enables interaction with the database.

Does SQL support programming language features?

SQL is a command language, not a programming language. It does not support programming constructs like loops or conditional statements. Instead, SQL consists of commands to query, update, and manage database data.

Difference between CHAR and VARCHAR2 datatype in SQL

Both CHAR and VARCHAR2 are used for character strings. CHAR is for fixed-length strings, while VARCHAR2 is for variable-length strings. For instance, CHAR(5) always stores 5 characters, whereas VARCHAR2(5) can store up to 5 characters but can store fewer.

What is data definition language?

Data Definition Language (DDL) includes commands like CREATE, DROP, and ALTER, which define the structure of the database and its objects.

What is data manipulation language?

Data Manipulation Language (DML) is used to access and manipulate data in the database. It includes commands like INSERT, DELETE, SELECT, and UPDATE.

What is a view in SQL?

A view is a virtual table based on the result set of an SQL query. It can include all or selected rows from one or more tables. For example, if you're a manager, you can create a view that shows the sales performance of your team. Views are created using the CREATE VIEW statement.

What is a foreign key?

A foreign key is a field that uniquely identifies a row in another table, creating a link between the two tables. It references the primary key of the other table.

What are table and field?

A table is a collection of related data entries that consists of rows and columns. A field is a single piece of data in a record (row) within a table.

What is the primary key?

A primary key is a unique identifier for a record in a table. It ensures that each record can be uniquely identified and no duplicate values exist.

What is a default constraint?

A default constraint provides a default value for a column when no value is specified. It automatically fills the column with the specified default value.

What is normalization?

Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves dividing large tables into smaller, more manageable tables and defining relationships between them. To put it simply, normalization is like organizing your wardrobe by separating your clothes into different drawers based on their type (shirts, pants, socks, etc.) and color.

What is denormalization?

Denormalization is the process of adding redundancy to a database to improve read performance by reducing the need for joins. It is applied after normalization.

What is a query?

An SQL query is used to retrieve data from the database. Efficient queries are crucial for optimal database performance.

What is a subquery?

A subquery is a query nested within another query. It is used to perform operations that require multiple steps within a single SQL statement.

Different operators in SQL

SQL supports various operators, including arithmetic operators (+, -, *, /), logical operators (AND, OR, NOT), and comparison operators (=, <>, <, >, <=, >=).

What is a constraint?

Constraints are rules applied to columns in a table to enforce data integrity. Examples include PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK constraints.

What is data integrity?

Data integrity ensures that data in a database is accurate and consistent. It is maintained through the use of constraints, keys, and normalization.

What is auto increment?

Auto increment is a feature that automatically generates a unique identifier for new records inserted into a table. It is commonly used for primary keys.

What is MySQL collation?

Collation in MySQL is a set of rules used to compare and sort characters in a particular character set. Each character set can have multiple collations, but only one default collation.

What are user-defined functions?

User-defined functions (UDFs) are custom functions created by users to extend SQL functionality. They can be used in SELECT lists, WHERE clauses, and other SQL expressions.

Intermediate SQL Interview Questions and Answers

What is an index?

An index is a database object that improves the speed of data retrieval operations on a table. It creates a quick lookup table to find rows efficiently.

Types of indexes in SQL

There are several types of indexes in SQL, including:

- Clustered Index: Physically rearranges the data in the table to match the Index.

- Non-Clustered Index: Creates a separate structure that points to the data in the table.

- Unique Index: Ensures all values in the indexed column are unique.

What is a join?

A join is an SQL operation used to combine rows from two or more tables based on a related column between them.

Different types of joins

- INNER JOIN: Returns records that have matching values in both tables.

- LEFT JOIN (LEFT OUTER JOIN): Returns all records from the left table and matched records from the right table.

- RIGHT JOIN (RIGHT OUTER JOIN): Returns all records from the right table and matched records from the left table.

- FULL JOIN (FULL OUTER JOIN): Returns all records when there is a match in either left or right table.

What is a stored procedure?

A stored procedure is a set of SQL statements that can be stored and executed on the database server. They help in code reuse and improving performance.

Advantages of stored procedures

- Performance: Reduces network traffic and improves performance.

- Security: Provides a layer of security as users can execute the procedure without knowing the underlying SQL code.

- Maintain ability: Easier to maintain as changes can be made in one place.

What are triggers?

Triggers are special types of stored procedures that automatically execute in response to certain events on a table, such as INSERT, UPDATE, or DELETE.

Types of triggers in SQL

- Before Triggers: Execute before the triggering event.

- After Triggers: Execute after the triggering event.

- Instead of Triggers: Execute in place of the triggering event.

What is a cursor?

A cursor is a database object that allows row-by-row processing of result sets. It is used when operations need to be performed on each row individually.

Difference between a cursor and a trigger

A cursor is used for row-by-row processing of data, while a trigger automatically executes in response to specific events in a table.

Advanced SQL Interview Questions and Answers

What is a transaction?

A transaction is a sequence of SQL operations performed as a single logical unit of work. Transactions ensure data integrity and consistency.

ACID properties in SQL

ACID stands for Atomicity, Consistency, Isolation, and Durability, which are key properties that ensure reliable transaction processing.

What is database locking?

Database locking is a mechanism to prevent simultaneous access to data by multiple users, ensuring data consistency and integrity.

Types of locks in SQL

- Shared Lock: Allows multiple transactions to read but not modify data.

- Exclusive Lock: Prevents other transactions from reading or modifying data.

- Update Lock: Prevents a deadlock situation by allowing multiple transactions to read data but only one to modify it.

What is isolation level?

Isolation level determines how transaction integrity is visible to other transactions and affects locking behavior.

Different isolation levels in SQL

- Read Uncommitted: Allows reading uncommitted data, leading to dirty reads.

- Read Committed: Prevents dirty reads but allows non-repeatable reads.

- Repeatable Read: Prevents dirty and non-repeatable reads but allows phantom reads.

- Serializable: Prevents dirty, non-repeatable, and phantom reads, ensuring complete isolation.

What is a deadlock?

A deadlock occurs when two or more transactions block each other by holding locks and requesting locks held by the other transaction.

How to handle deadlocks in SQL

Deadlocks can be handled by implementing proper transaction management techniques, such as:

- Timeouts: Automatically abort transactions that wait too long.

- Deadlock detection: Identify and resolve deadlocks by aborting one of the transactions.

- Locking order: Ensure transactions acquire locks in a consistent order.

What is partitioning in SQL?

Partitioning is a database optimization technique that divides large tables into smaller, more manageable pieces without changing the application logic.

Types of partitioning

- Range Partitioning: Divides data based on a range of values.

- List Partitioning: Divides data based on a list of values.

- Hash Partitioning: Divides data based on a hash function.

- Composite Partitioning: Combines multiple partitioning methods.

What is the difference between SQL and MySQL?

SQL is a standard language for querying databases, while MySQL is a relational database management system (RDBMS) that uses SQL.

How can I optimize my SQL queries?

Optimize SQL queries by:

- Using proper indexes.

- Avoiding unnecessary columns in SELECT statements.

- Using JOINs instead of subqueries.

- Analyzing and optimizing query execution plans.

What are the common SQL interview mistakes to avoid?

Common mistakes include:

- Lack of understanding of basic concepts.

- Inability to write optimized queries.

- Overlooking data integrity and normalization principles.

- Failing to explain the logic behind complex queries.

How important are SQL certifications?

SQL certifications can enhance your resume by demonstrating your expertise and commitment to learning. They can be particularly useful for beginners or those looking to transition into database roles.

What resources can help me prepare for SQL interviews?

Conclusion

Preparing for an SQL interview involves understanding a wide range of concepts, from basic definitions to advanced transaction management. By mastering these topics, you'll be well-equipped to tackle questions on data manipulation, database design, and performance optimization. Remember to practice writing and optimizing SQL queries, as real-world scenarios often involve complex data operations. Good luck with your interview preparation!

Useful resources include:

- Online courses (e.g., Coursera, Udemy). JOIN TELEGRAM CHANNEL FOR FREE UDEMY COURSES

- SQL practice platforms (e.g., LeetCode, HackerRank).

- Official documentation and tutorials from database vendors.

- Books like "SQL for Dummies" and "SQL Performance Explained". FREE EBOOKS AVAILABLE HERE CLICK HERE