Interview Questions & Answer for Data Analyst Profile

 Interview Questions & Answers for Data Analyst Profile

1. What is SQL and what are its main components?

Answer: SQL stands for Structured Query Language, and it is a programming language used to manage and manipulate relational databases. The main components of SQL are:

Data Definition Language (DDL): Used to create and modify database structures, such as tables and indexes.

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

Data Control Language (DCL): Used to grant or revoke permissions and control access to the database.

2. Explain the differences between INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.

Answer:

INNER JOIN: Returns only the rows that have matching values in both tables being joined.

LEFT JOIN: Returns all the rows from the left (or first) table and the matching rows from the right (or second) table. If there is no match, NULL values are returned for the right table columns.

RIGHT JOIN: Returns all the rows from the right (or second) table and the matching rows from the left (or first) table. If there is no match, NULL values are returned for the left table columns.

FULL JOIN: Returns all the rows from both tables, including the unmatched rows. If there is no match, NULL values are returned for the unmatched columns.

3. How do you handle NULL values in SQL?

Answer: NULL values can be handled in SQL using various techniques, such as:

IS NULL: Checks if a value is NULL.

IS NOT NULL: Checks if a value is not NULL.

COALESCE: Replaces NULL values with a specified default value.

IFNULL: Replaces NULL values with a specified default value (MySQL).

NVL: Replaces NULL values with a specified default value (Oracle).

NULLIF: Compares two expressions and returns NULL if they are equal; otherwise, returns the first expression.

Example: To select all employees with a non-NULL salary:

SELECT * FROM employees WHERE salary IS NOT NULL;

4. What are the differences between a primary key and a foreign key?

Answer: Primary Key: A primary key is a column or a combination of columns that uniquely identifies each row in a table. It enforces entity integrity and ensures that there are no duplicate or NULL values. Only one primary key is allowed per table.

Foreign Key: A foreign key is a column or a combination of columns that establishes a link or relationship between two tables. It references the primary key of another table and ensures referential integrity. Multiple foreign keys can exist in a table.

Example: In the "Employees" table, the "employee_id" column could be the primary key, uniquely identifying each employee. In the "Orders" table, the "employee_id" column could be a foreign key, referencing the "employee_id" in the "Employees" table.

5. Explain the concept of normalization and its importance in database design.

Answer: Normalization is the process of organizing data in a database to eliminate redundancy and dependency issues. It involves dividing larger tables into smaller ones and establishing relationships between them. The main goals of normalization are to minimize data duplication, ensure data consistency, and improve data integrity. Normalization is essential in database design as it helps to optimize storage, reduce update anomalies, and improve overall performance.

There are different normal forms, such as First Normal Form (1NF), Second Normal Form (2NF), Third Normal Form (3NF), and so on, each with specific rules and criteria to meet for achieving better data organization.

Example: Let's consider a table called "Customers" that stores customer data. In its initial form, the table may have redundant data like customer addresses being repeated for each order. By normalizing the table into separate tables for customers and orders, we eliminate redundant data and establish a relationship between them.

Before normalization:

Customers Table:

Customer ID Customer Name Address

1                     John                123 Main St

2                     Mary                456 Elm St

3                     David                123 Main St

After normalization:

Customers Table:

Customer ID Customer Name

    1                         John

    2                         Mary

    3                         David

Orders Table:

Order ID Customer ID Order Date

        1         1         2022-05-10

        2         2         2022-06-15

        3         1         2022-07-20