If you want to become a software developer, backend engineer, data analyst, or even work in the data science and AI field, having SQL knowledge is crucial. It is very important to understand databases and how to manage data using SQL, RDBMS, etc. Collecting and managing data is essential for every company, and no software, website, or company operates without collecting data. Data helps them understand their customers, make complex decisions, and improve their products and services.
SQL Interview Questions for Freshers
1. What is SQL?
SQL stands for Structured Query Language. It is a language used to manage and manipulate relational databases. SQL helps you perform tasks like retrieving, inserting, updating, and deleting data from a database.
2. What is a Database?
A database is a collection of organized data So it can be easily accessed, managed, and updated. It stores data in tables, which consist of rows and columns.
3. What is a Table in SQL?
A table in SQL is a collection of data organized in tabular format like rows and columns. Each table represents a different type of data and contains related information. For example, a table named Employees can store data about employees with columns for their names, IDs, and positions.
4. What do you mean by Primary Key?
A primary key is used to uniquely identify the each record in a table. It ensures that each record is unique and It helps maintain the integrity of the data.
5. What do you mean by Foreign Key?
A foreign key field in one table links to the primary key of another table. It is used to establish a connection or link between two tables to maintain the data integrity. For example, In a database with two tables called "Customers" and "Orders", a foreign key in a Orders table can link to the primary key in the Customers table. So we can check which customers placed which orders.
6. What is a SQL Join?
A SQL join is used to combine rows from two or more tables based on a related column between them. There are different types of joins, such as INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN, and each serves different purposes to retrieve related data.
7. What is the difference between INNER JOIN and LEFT JOIN?
INNER JOIN returns only the rows with matching values in both tables. LEFT JOIN returns all rows from the left table and only matched rows from the right table. If there is no match, the result will include NULL values for columns from the right table.
8. Explain All SQL Joins?
INNER JOIN: It returns only rows with matching values in both tables.
LEFT JOIN: It returns all rows from the left table and only matching rows from the right table.
RIGHT JOIN: It returns all rows from the right table and only matching rows from the left table.
FULL JOIN: It returns all rows if there is a match in either a right table or a left table.
9. Why do we use SQL Query?
A SQL query is a command used to get data or records from a database. It can be used to retrieve, update, delete, or insert data. The most common SQL query is SELECT, which retrieves data from a table.
10. What is a Subquery?
A subquery is a query within another query. It is used to perform operations that depend on the results of another query. We can use subqueries with SELECT, INSERT, UPDATE, or DELETE statements.
11. What is the use of Index?
An index is a database object that is used to improve the speed of data retrieval operations. It works similarly like a page index in a book using which we can quickly find the required page directly without going through each page.
12. What is the difference between UNION and UNION ALL?
UNION combines the results of two or more SELECT queries and removes duplicate rows. UNION ALL combines results and includes all rows, even duplicates.
13. Why do we use the Stored Procedure?
A stored procedure is a precompiled collection of SQL statements or code that can be executed as a single unit. It is used to increase the code reusability, performance, and security. For example, instead of writing the same SQL code multiple times, we can store that repetitive code as a procedure and simply call the procedure when needed.
14. What is a Trigger?
A trigger is a special type of stored procedure that automatically runs in response to certain events on a table, such as INSERT, UPDATE, or DELETE operations. Triggers are used to enforce business rules and maintain data integrity.
15. What is Normalization?
Normalization is the process of structuring data in a database to minimize redundancy and enhance data integrity. By breaking down large tables into smaller, related ones and establishing clear relationships between them, normalization ensures that your database is efficient, organized, and easier to maintain.
16. What is Denormalization?
Denormalization is the process of combining tables to improve query performance. It can involve adding redundant data or merging tables to reduce the number of joins needed, which can speed up read operations.
17. What is a Constraint in SQL?
A constraint is a rule applied to columns or tables to enforce data integrity. Common constraints are NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK. For example, We can use NOT NULL constraint to make sure that the column should not contain null values.
18. What is the difference between DELETE and TRUNCATE?
DELETE removes rows from a table based on a condition But here, deleted data can be rolled back. TRUNCATE removes all rows from a table quickly But here, data cannot be rolled back, as it does not log individual row deletions.
19. What is an Aggregate Function?
Aggregate functions are used to perform calculations on multiple rows of data and return a single value. Common aggregate functions are COUNT, SUM, AVG, MIN, and MAX.
20. What is a Transaction?
A transaction is a sequence of SQL operations that are executed as a single unit. Transactions ensure that either all operations are completed successfully or none are applied, maintaining database consistency.
For Example: Imagine you are transferring money to your friend's account. The transaction ensures that both the debit from your account and the credit to your friend's account happen together. If there is a problem with either of these steps, no operation should be performed.
21. What are SQL Data Types?
SQL data types define the kind of data that can be stored in a column. Common data types are INTEGER, VARCHAR, DATE, and BOOLEAN.
22. What is the purpose of the GROUP BY clause?
The GROUP BY clause groups rows that have the same values in specified columns into summary rows. It helps in grouping data based on one or more columns, so you can perform aggregate functions like COUNT, SUM, or AVG on those groups
23. What is a JOIN condition?
A JOIN condition is used to combine rows from two or more tables based on a related column between them. It specifies how to match rows in one table with rows in another to retrieve related data.
24. What is a CTE (Common Table Expression)?
A CTE is a temporary result set defined within a SELECT, INSERT, UPDATE, or DELETE statement. It can be used to simplify complex queries and make them more readable.
25. What is the purpose of the HAVING clause?
The HAVING clause is used to filter the results of a GROUP BY query based on aggregate values. It is similar to the WHERE clause but is used for grouped data.
26. What is the purpose of the ORDER BY clause?
The ORDER BY clause is used to sort the result set of a query by one or more columns. It can sort data in ascending (ASC) or descending (DESC) order.
27. What is a Database Schema?
A database schema is the structure that defines the organization of data in a database. It includes the tables, columns, data types, and relationships between tables.
28. What is Data Integrity?
Data integrity refers to the accuracy and consistency of data in a database. It ensures that data is correct, reliable, and maintained according to predefined rules and constraints.
29. What is SQL Injection?
SQL injection is a security vulnerability that allows an attacker to execute malicious SQL queries by injecting them into input fields. It can lead to unauthorized access or manipulation of data.
30. What is a View?
A view is a virtual table based on the result of a SELECT query. It does not store data itself but provides a way to access data from one or more tables through a single, simplified interface.
I hope these SQL interview questions give you an idea to prepare for your next interview. Remember, mastering SQL is a long journey, and the more you practice, the better you'll get. Whether you're just starting out or brushing up on your skills, these questions will help you gain the confidence you need to tackle any SQL challenge. Thanks for reading, and happy coding!
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