Top 12 SQL Interview Questions: A Practical Guide With Solution

Introduction

These Are the top 12 SQL Interview Questions for 2024

1. What is SQL?
   • SQL stands for Structured Query Language, used for managing and manipulating relational databases.
2. What are the types of SQL commands?
   • SQL commands are broadly classified into four categories: Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), and Transaction Control Language (TCL).
3. Explain the difference between SQL and MySQL.
   • SQL (Structured Query Language) is a standardized language used for managing relational databases, whereas MySQL is an open-source Relational Database Management System (RDBMS) that uses SQL as its language.
4. What is a primary key in SQL?
   • A primary key is a unique identifier for each record in a table. It ensures data integrity and prevents duplicate entries.
5. What is the purpose of the SELECT statement?
   • The SELECT statement is used to retrieve data from a database. It allows users to specify which columns or fields they want to retrieve and from which tables.
6. Explain the difference between WHERE and HAVING clauses. # This is a Basic Level Mostly Asked SQL Interview question..
   • WHERE clause is used to filter records before the data is grouped, while the HAVING clause is used to filter records after the data is grouped.
7. What is a JOIN in SQL?
   • This is Mostly Asked SQL Interview Questions in every SQL interview Round in Data Analyst. JOIN combines rows from two or more tables based on a related column between them.
   • Consider two hypothetical tables, “Employees” and “Departments”:
   • Employees Table:
     • • EmployeeID EmployeeName DepartmentID
       • 1 Ramesh 1
       • 2 Pramod 1
       • 3 Shourya 2
       • 4 Mohan 3
       • 5 Neha Null
       • Departments Table:
       • DepartmentID DepartmentName
       • 1 Sales
       • 2 Marketing
       • 3 HR
       • 4 Finance

INNER JOIN: Many times Asked to JOINS in Mostly SQL Interview Questions in 2023

• Returns rows when there is a match in both tables based on the specified condition

SELECT Employees.EmployeeID, Employees.EmployeeName, Departments.DepartmentName
FROM Employees
INNER JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID;

LEFT JOIN:

• Returns all rows from the left table (Employees) and matching rows from the right table (Departments).

SELECT Employees.EmployeeID, Employees.EmployeeName, Departments.DepartmentName
FROM Employees
LEFT JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID;

RIGHT JOIN:

• Returns all rows from the right table (Departments) and matching rows from the left table (Employees).

SELECT Employees.EmployeeID, Employees.EmployeeName, Departments.DepartmentName
FROM Employees
RIGHT JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID;

FULL JOIN:

• Returns all rows when there is a match in the Employees or Departments table.

SELECT Employees.EmployeeID, Employees.EmployeeName, Departments.DepartmentName
FROM Employees
FULL JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID;

CROSS JOIN:

• Returns the Cartesian product of both tables, resulting in all possible combinations.

SELECT Employees.EmployeeID, Employees.EmployeeName, Departments.DepartmentName
FROM Employees
CROSS JOIN Departments;

8. What are SQL constraint#s? #Mostly asked SQL interview questions in 2023 in Basic Round Interview

Constraints are rules applied to columns to enforce data integrity and ensure the accuracy and reliability of data in a database. Examples include NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK constraints

9. What is the difference between DELETE and TRUNCATE commands? This Mostly Asked SQL Interview Questions

• The DELETE command removes specific rows from a table, while the TRUNCATE command removes all rows from a table, resetting table data and structure.

10. Explain the concept of NULL in SQL.

NULL is a special value in SQL representing the absence of a value. It differs from zero or an empty string and signifies that the value is unknown or not applicable.

11. what is the order of execution in SQL? #This is the most asked SQL Interview Question in 2023

The general order of execution in SQL is as follows:

First: FROM: The FROM clause identifies the tables or views from which data will be retrieved.

2nd: WHERE: The WHERE clause filters rows based on specified conditions, restricting the result set to rows that meet the given criteria.

3rd: GROUP BY: If specified, the GROUP BY clause is used to group rows sharing common values into summary rows.

4th: HAVING: The HAVING clause filters grouped rows based on specified conditions, similar to the WHERE clause but applied after GROUP BY.

5th: SELECT: The SELECT clause specifies the columns to be included in the result set and performs any calculations or transformations on the data.

6th: ORDER BY: The ORDER BY clause sorts the result set based on specified columns or expressions, arranging the rows in ascending or descending order.

7th: LIMIT/OFFSET (or FETCH/FIRST): These clauses, available in some SQL dialects, control the number of rows returned or the starting point for fetching rows, enabling pagination or limiting result set size.

12. What is the Window Function?

Hello friends Window functions come under analytics Functions in SQL, You can not crack any SQL Interview without these functions. Window functions in SQL are powerful analytical tools that operate on a specific set of rows within a query result, known as a window or frame. These functions perform calculations, aggregations, and comparisons across this defined window, offering various use cases:

Consider a hypothetical table “Sales” with columns: OrderID, ProductID, Date, and Amount.

Calculating Row Number:

SELECT OrderID, ProductID, Date, Amount,
ROW_NUMBER() OVER (ORDER BY Date) AS RowNumber
FROM Sales;

=>This assigns a unique row number to each row based on the order of dates.

Calculating Running Total:

SELECT OrderID, ProductID, Date, Amount,
SUM(Amount) OVER (ORDER BY Date) AS Running_Total
=> Computes the running total of the “Amount” column based on the date order.

Determining Ranks:

SELECT OrderID, ProductID, Date, Amount,
RANK() OVER (PARTITION BY ProductID ORDER BY Amount DESC) AS ProductRank
FROM Sales;

=> Provides the ranking of products based on their amounts within each partition of ProductID.

Calculating Moving Averages:

SELECT OrderID, ProductID, Date, Amount,
AVG(Amount) OVER (ORDER BY Date ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) AS MovingAverage
FROM Sales;

=> Computes the moving average of the “Amount” column over the current row’s window and the two preceding rows based on the date order.

Identifying Lead and Lag Values: Important Conceot of Mostly Asked SQL Interview Questions

SELECT OrderID, ProductID, Date, Amount,
LAG(Amount) OVER (ORDER BY Date) AS PreviousAmount,
LEAD(Amount) OVER (ORDER BY Date) AS NextAmount
FROM Sales;

=> Retrieves the previous and next amounts based on the date order.

These are foundational Mostly Asked SQL Interview Questions commonly asked in interviews, offering a glimpse into a candidate’s understanding of database concepts and SQL operations. After Completing These Mostly Asked SQL Interview Questions You should practice These Complex Interview Questions.

Conclusion

In conclusion, the compilation of the top 12 SQL interview questions for 2023-24 provides valuable insight into the skills and knowledge sought by employers in the ever-evolving landscape of data management and analytics. These questions cover a spectrum of SQL topics, from fundamental querying to more advanced concepts such as optimization and normalization.

As candidates prepare for SQL interviews, a thorough understanding of these SQL interview questions not only demonstrates proficiency in database management but also reflects an awareness of industry trends and requirements. Ultimately, mastering these SQL interview questions equips candidates with a competitive edge, ensuring they are well-prepared to navigate the complexities of modern data-centric roles in the coming year.