SQL Query Interview Questions for Freshers
Tags: sql, sqlserver, database, datascience
Published: 2023-11-02 08:01:38

Are you a fresher looking to break into the world of database management and SQL querying? Congratulations, you're on the right track! SQL (Structured Query Language) is a fundamental tool for managing and extracting data from databases. 

Many employers value this skill, and SQL query interview questions are common in job interviews for roles like data analyst, database administrator, and software developer. In this blog, we will explore a wide range of SQL query interview questions suitable for freshers. We will provide explanations and tips to help you ace your interview.

## Introduction

Before we dive into the interview questions, let's start with a brief introduction to SQL. SQL is a specialized language designed for managing and manipulating relational databases. It allows you to perform various operations on data, such as retrieving, updating, inserting, and deleting records from a database. 

In an interview setting, you'll likely face questions that test your knowledge of SQL's basic syntax, data manipulation, and database design principles.

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## SQL Basics

### 1. What is SQL, and what is its primary use?

**SQL** stands for **Structured Query Language**, and its primary use is for managing and querying data in relational database systems.

### 2. Name some popular relational database management systems (RDBMS) that use SQL.

- **MySQL**
- **Oracle Database**
- **Microsoft SQL Server**
- **PostgreSQL**
- **SQLite**

### 3. Explain the difference between SQL and NoSQL databases.

SQL databases are relational databases that use structured tables to store data, while NoSQL databases are non-relational and can store data in various formats, such as JSON or XML. SQL databases are good for structured data, while NoSQL databases are more flexible for handling unstructured data.

### 4. What are the common data types in SQL?

Some common data types in SQL include:
- **INT** (Integer)
- **VARCHAR** (Variable-length character string)
- **DATE** (Date)
- **DECIMAL** (Decimal number)

### 5. How do you comment in SQL?

In SQL, you can use `--` for single-line comments and `/* */` for multi-line comments.

## SQL Querying

### 6. What is a SQL SELECT statement used for?

A **SELECT** statement is used to retrieve data from a database. It allows you to specify the columns you want to retrieve and apply filters to narrow down the results.

### 7. Write a basic SQL SELECT statement to retrieve all columns from a table named "employees."

```sql
SELECT * FROM employees;
```

### 8. How do you retrieve only specific columns from a table?

To retrieve specific columns from a table, list the column names after the **SELECT** keyword.

```sql
SELECT first_name, last_name, email FROM employees;
```

### 9. Explain the difference between the WHERE and HAVING clauses.

The **WHERE** clause is used to filter rows before grouping or aggregating data, while the **HAVING** clause is used to filter groups of rows after data has been grouped using GROUP BY.

### 10. What is the ORDER BY clause used for?

The **ORDER BY** clause is used to sort the result set based on one or more columns in ascending (ASC) or descending (DESC) order.

```sql
SELECT product_name, price FROM products ORDER BY price DESC;
```

### 11. How can you eliminate duplicate rows from a result set?

You can use the **DISTINCT** keyword to eliminate duplicate rows from the result set.

```sql
SELECT DISTINCT department FROM employees;
```

### 12. What is a SQL JOIN, and how is it used?

A SQL **JOIN** is used to combine rows from two or more tables based on a related column between them. Common types of joins include **INNER JOIN**, **LEFT JOIN**, **RIGHT JOIN**, and **FULL OUTER JOIN**.

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### 13. Explain the difference between INNER JOIN and LEFT JOIN.

- **INNER JOIN**: Returns only the rows where there is a match in both tables.
- **LEFT JOIN**: Returns all rows from the left table and the matched rows from the right table. If there's no match, NULL values are returned.

### 14. How do you write a SQL query to find the total number of employees in each department?

```sql
SELECT department, COUNT(employee_id) AS employee_count
FROM employees
GROUP BY department;
```

### 15. What is a SQL subquery, and when is it used?

A SQL subquery is a query nested within another query. It's used to retrieve data necessary for the main query or to filter results based on a condition.

### 16. Write a SQL query to find employees who earn more than the average salary.

```sql
SELECT first_name, last_name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);
```

### 17. Explain the purpose of the SQL UNION operator.

The SQL **UNION** operator is used to combine the result sets of two or more SELECT statements. It removes duplicate rows by default.

```sql
SELECT employee_id, first_name FROM employees
UNION
SELECT student_id, student_name FROM students;
```

## Data Modification

### 18. What is the SQL INSERT statement used for?

The **INSERT** statement is used to add new rows to a table.

### 19. Write a SQL INSERT statement to add a new employee to the "employees" table.

```sql
INSERT INTO employees (first_name, last_name, email, hire_date)
VALUES ('John', 'Doe', 'john.doe@example.com', '2023-11-02');
```

### 20. Explain the purpose of the SQL UPDATE statement.

The **UPDATE** statement is used to modify existing records in a table. You specify which columns to update and provide the new values.

```sql
UPDATE employees
SET salary = 60000
WHERE last_name = 'Doe';
```

### 21. How do you delete records from a table using the SQL DELETE statement?

The **DELETE** statement is used to remove rows from a table based on a specified condition.

```sql
DELETE FROM employees
WHERE employee_id = 101;
```

## Database Design

### 22. What is a database schema?

A **database schema** is a logical container for database objects such as tables, views, and indexes. It helps organize and manage the database structure.

### 23. Explain the concept of primary keys in a database.

A **primary key** is a unique identifier for each row in a table. It ensures that each record is unique and can be used to establish relationships between tables.

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### 24. What is a foreign key, and how is it used?

A **foreign key** is a column or a set of columns in a table that is used to establish a link between the data in two tables. It enforces referential integrity by ensuring that the values in the foreign key column exist in the primary key column of another table.

### 25. What are indexes in a database, and why are they important?

**Indexes** are data structures that improve the speed of data retrieval operations on a database table. They allow you to quickly locate rows that match a search condition.

### 26. Explain the concept of normalization in database design.

**Normalization** is the process of organizing data in a database to reduce data redundancy and improve data integrity.

 It involves breaking down tables into smaller, related tables to avoid data anomalies.

## Advanced SQL

### 27. What is a SQL stored procedure, and why is it used?

A **stored procedure** is a precompiled set of one or more SQL statements that can be executed as a single unit. It is used to encapsulate complex logic and make database operations more efficient.

### 28. How do you create a stored procedure in SQL?

To create a stored procedure, you can use the **CREATE PROCEDURE** statement. Here's an example:

```sql
CREATE PROCEDURE sp_GetEmployeeName
@employee_id INT
AS
BEGIN
    SELECT first_name, last_name
    FROM employees
    WHERE employee_id = @employee_id;
END;
```

### 29. What is SQL injection, and how can it be prevented?

**SQL injection** is a malicious technique where an attacker inserts harmful SQL code into a query. To prevent it, use parameterized queries and input validation to sanitize user inputs.

### 30. What is a SQL trigger, and when is it used?

A **SQL trigger** is a set of actions that are automatically performed when a specific event occurs in a database table. It is used to enforce business rules or maintain data integrity.

## Tips for Acing Your SQL Interview

- **Practice**: Regularly practice writing SQL queries to improve your skills.
- **Understand the Basics**: Ensure you have a solid understanding of SQL fundamentals.
- **Know Your RDBMS**: Be familiar with the specific RDBMS your potential employer uses.
- **Think Aloud**: During the interview, talk through your thought process when solving problems.
- **Ask Questions**: If you're unsure about a question, don't hesitate to ask for clarification.
- **Show Your Work**: When writing queries on a whiteboard or paper, make sure they are legible and well-organized.
- **Review Common Functions**: Familiarize yourself with common SQL functions and keywords.

## Conclusion

Preparing for SQL query interview questions as a fresher can be a challenging yet rewarding experience. With the right knowledge and practice, you can confidently navigate these questions and impress your potential employers. Remember to brush up on your SQL basics, understand data manipulation, and be ready to discuss database design and advanced SQL concepts. Good luck with your interviews, and may your SQL skills shine bright!