Top 20 Most Asked Interview Questions For Data Analyst / Business Analyst Profile

 Top 20 Most Asked Interview Questions For Data Analyst / Business Analyst Profile

What is SQL, and what are its main components?

SQL (Structured Query Language) is a programming language used for managing and manipulating relational databases. It consists of four main components:

Data Definition Language (DDL): Used to define and modify the structure of the database objects, such as tables, indexes, and constraints.

Data Manipulation Language (DML): Used to retrieve, insert, update, and delete data from the database tables.

Data Control Language (DCL): Used to control access to the database objects by granting or revoking permissions.

Transaction Control Language (TCL): Used to manage transactions, allowing for atomicity, consistency, isolation, and durability.

What is the difference between SQL and NoSQL databases?

SQL databases are based on a relational model and use structured query language, while NoSQL databases are non-relational and do not use SQL. Some key differences include:

Data Model: SQL databases use a predefined schema and tables with fixed columns, while NoSQL databases have a flexible schema and use various data models like key-value, document, columnar, or graph.

Scalability: NoSQL databases are designed to scale horizontally and handle large amounts of unstructured data, whereas SQL databases are typically vertically scalable.

ACID Support: SQL databases provide ACID (Atomicity, Consistency, Isolation, Durability) properties, ensuring data integrity, while NoSQL databases may sacrifice some of these properties for scalability and performance.

What are the different types of joins in SQL? Explain each.

The different types of joins in SQL are:

INNER JOIN: Retrieves records that have matching values in both tables being joined.

LEFT JOIN: Retrieves all records from the left (first) table and matching records from the right (second) table.

RIGHT JOIN: Retrieves all records from the right (second) table and matching records from the left (first) table.

FULL OUTER JOIN: Retrieves all records from both tables, including non-matching records.

CROSS JOIN: Returns the Cartesian product of both tables, producing all possible combinations of rows.

What is the difference between UNION and UNION ALL?

UNION combines the result sets of two or more SELECT statements, removing duplicates. UNION ALL also combines the result sets but includes all rows, including duplicates.

How do you eliminate duplicate rows from a result set in SQL?

To eliminate duplicate rows from a result set, you can use the DISTINCT keyword in the SELECT statement. For example:

Question: How do you eliminate duplicate rows from a result set in SQL?

Answer: You can use the DISTINCT keyword in the SELECT statement. For example: SELECT DISTINCT column1, column2 FROM table;

Explain the difference between a primary key and a foreign key.

Question: Explain the difference between a primary key and a foreign key.

Answer: A primary key is a unique identifier for a record in a table. It ensures the uniqueness and integrity of the data within the table. A foreign key is a field in one table that refers to the primary key of another table, establishing a relationship between the two tables.

What is the purpose of an index in a database?

Question: What is the purpose of an index in a database?

Answer: The purpose of an index in a database is to improve the performance of data retrieval operations. It works like an index in a book, allowing the database to quickly find the required data by creating a separate structure that contains a sorted copy of the indexed columns.

What is a subquery, and how is it different from a regular query?

Question: What is a subquery, and how is it different from a regular query?

Answer: A subquery, also known as an inner query or nested query, is a query embedded within another query. It is used to retrieve data based on the results of another query. A regular query is a standalone query used to retrieve data from one or more tables.

How do you perform data aggregation in SQL? Provide some examples.

Question: How do you perform data aggregation in SQL? Provide some examples.

Answer: Data aggregation in SQL is done using aggregate functions such as COUNT, SUM, AVG, MIN, and MAX. For example:

SELECT COUNT(*) FROM table;

SELECT SUM(sales) FROM table;

SELECT AVG(age) FROM table;

What are the different data types in SQL? Explain a few commonly used ones.

Question: What are the different data types in SQL? Explain a few commonly used ones.

Answer: SQL supports various data types, including:

INT: Represents integer values.

VARCHAR: Variable-length character strings.

DATE: Stores date values.

DECIMAL: Numeric data type for fixed-point numbers.

BOOLEAN: Represents true or false values.

Question: How do you handle NULL values in SQL queries?

Answer: NULL values can be handled in SQL queries using the IS NULL and IS NOT NULL operators. These operators are used to check if a column contains a NULL value or not. For example:

SELECT * FROM table WHERE column IS NULL;

SELECT * FROM table WHERE column IS NOT NULL;

Question: Explain the concept of normalization and its different forms.

Answer: Normalization is the process of organizing data in a database to minimize redundancy and dependency. It involves dividing a database into multiple tables and defining relationships between them. The different forms of normalization include:

First Normal Form (1NF): Eliminates duplicate data by organizing information into tables with atomic values.

Second Normal Form (2NF): Ensures that non-key attributes depend on the entire key and not just part of it.

Third Normal Form (3NF): Removes transitive dependencies, where a non-key attribute depends on another non-key attribute.

Question: What is the difference between a clustered and a non-clustered index?

Answer: A clustered index determines the physical order of the data rows in a table. Each table can have only one clustered index. On the other hand, a non-clustered index is a separate structure that contains a copy of the indexed columns and a reference to the corresponding data rows. A table can have multiple non-clustered indexes.

Question: What is the purpose of the GROUP BY clause in SQL? Provide an example.

Answer: The GROUP BY clause is used to group rows based on one or more columns and perform aggregate functions on each group. For example:

SELECT department, COUNT(*) FROM employees GROUP BY department;

This query groups employees by their departments and returns the count of employees in each department.

Question: How do you retrieve the top 'n' records from a table in SQL?

Answer: To retrieve the top 'n' records from a table, you can use the LIMIT clause in MySQL or PostgreSQL, or the TOP clause in SQL Server. For example:

MySQL/PostgreSQL: SELECT * FROM table LIMIT n;

SQL Server: SELECT TOP n * FROM table;

Question: Explain the difference between a stored procedure and a function.

Answer: A stored procedure is a set of SQL statements that are stored in the database and can be executed multiple times. It can have input and output parameters and does not necessarily return a value. On the other hand, a function is a database object that returns a value and is typically used in SQL expressions. Functions can be called from within queries.

Question: How do you perform data modification operations (INSERT, UPDATE, DELETE) in SQL?

Answer: Data modification operations in SQL are performed using the following statements:

INSERT: Inserts new records into a table.

UPDATE: Modifies existing records in a table.

DELETE: Deletes records from a table.

Question: What is the difference between the HAVING clause and the WHERE clause?

Answer: The WHERE clause is used to filter rows before grouping and aggregation, while the HAVING clause is used to filter groups after grouping and aggregation. The HAVING clause operates on the result of the GROUP BY clause.

Question: How do you perform data pivoting in SQL?

Answer: Data pivoting in SQL is achieved using the PIVOT function. It allows you to transform rows into columns based on a specific criteria. The PIVOT function requires an aggregate function and the columns to pivot. For example:

SELECT * FROM table PIVOT (SUM(value) FOR column_name IN (list_of_values));

Question: Explain the concept of transactions and ACID properties in SQL.

Answer: Transactions in SQL are a set of SQL statements that are executed as a single unit. ACID properties are the fundamental principles that ensure reliability and consistency of transactions:

Atomicity: Transactions are treated as a single, indivisible unit of work. They are either completed in their entirety or rolled back if any part fails.

Consistency: Transactions bring the database from one consistent state to another. They preserve data integrity and maintain any defined constraints.

Isolation: Transactions operate in isolation from each other. The intermediate states of a transaction are not visible to other concurrent transactions until the transaction is committed.

Durability: Once a transaction is committed, its changes are permanent and survive any subsequent system failures.