SQL (Structured Query Language) remains the backbone of relational database management. For freshers stepping into roles like Database Developer, Data Analyst, or Backend Engineer, a solid grasp of SQL fundamentals—and the ability to articulate them in interviews—is critical. This article explores 50 common SQL interview questions for beginners, complete with concise explanations to help you prepare effectively.
Table of Contents
Tips for SQL Interview Success
- Understand the fundamentals: Know data types, basic commands (SELECT, INSERT, UPDATE, DELETE), and constraint concepts.
- Practice SQL hands-on: Use free tools like SQLite, MySQL, or PostgreSQL sandboxes to run queries.
- Learn to read and write queries: Focus on writing clear, optimized queries and reading execution plans.
- Familiarize with real-world scenarios: Think in terms of tables such as Employees, Orders, or Products.
- Brush up on normalization: Understand 1NF, 2NF, 3NF, and why they matter.
- Prepare to explain your logic: Practice walking through your thought process when writing a query.
SQL Interview Questions for Freshers
What is SQL?
SQL stands for Structured Query Language, used to communicate with relational databases to perform tasks like querying, updating, and managing data.
Difference between DELETE, TRUNCATE, and DROP.
- DELETE removes rows and can be rolled back.
- TRUNCATE removes all rows, resets storage (cannot roll back in some systems).
- DROP deletes the entire table structure and data.
What are joins? List their types.
Joins combine rows from two or more tables. Types include: INNER, LEFT (OUTER), RIGHT (OUTER), FULL (OUTER), CROSS.
Explain a primary key and a foreign key.
- Primary key uniquely identifies each record in a table.
- Foreign key enforces a link between records in two tables.
What is normalization? Why normalize?
Normalization organizes data to reduce redundancy and improve integrity. It involves applying normal forms (1NF, 2NF, 3NF).
Describe indexes and their benefits.
Indexes speed up data retrieval by creating a data structure (e.g., B-tree). They can slow down writes but greatly enhance read performance.
What is a unique constraint?
Ensures all values in a column (or group of columns) are distinct.
Explain the difference between WHERE and HAVING.
- WHERE filters rows before grouping.
- HAVING filters groups after aggregation.
What are aggregate functions? Give examples.
Perform calculations on sets of values: COUNT(), SUM(), AVG(), MIN(), MAX().
How do you retrieve the top N rows in SQL?
Use LIMIT N (MySQL/PostgreSQL) or TOP N (SQL Server).
What is DBMS?
DBMS stands for Database Management System. It is software that manages databases, which are collections of data stored in an organized manner that allows for easy retrieval and manipulation.
Describe how to create and delete a database in SQL.
To create a database in SQL, you can use the CREATE DATABASE statement followed by the name of the database. To delete a database, you can use the DROP DATABASE statement followed by the name of the database.
How do you write a stored procedure in SQL?
To write a stored procedure in SQL, you can use the CREATE PROCEDURE statement followed by the name of the procedure and the code to be executed. The procedure can then be called by name from other SQL statements.
How do you use table aliases?
Table aliases in SQL are used to give a table a temporary name that can be used in place of its full name in a SQL statement. This can make the statement easier to read and write, especially when working with complex queries that involve multiple tables.
What is Schema?
A schema is a logical container for database objects such as tables, views, and procedures. It defines the structure of the database and provides a way to organize and group related objects.
What is a Table?
A table is a collection of related data stored in rows and columns. Each row represents a single record, while each column represents a specific attribute of the record.
What is Package?
A package is a collection of related database objects such as procedures, functions, and variables. It is used to group these objects together and provide a level of abstraction for the user.
What is a View?
In SQL, a view is a virtual table that is derived from one or more tables in a database. A view is not a physical table, but rather a saved query that can be treated as a table. Views can be used to simplify complex queries, present data in a more meaningful way, and provide an additional layer of security by restricting access to certain columns or rows of data.
Give some examples of SQL data statements.
Examples of SQL data statements include SELECT, INSERT, UPDATE, and DELETE. SELECT is used to retrieve data from a database, INSERT is used to add data to a database, UPDATE is used to modify existing data in a database, and DELETE is used to remove data from a database.
What are SQL connection statements?
SQL connection statements are used to connect to a database. Examples include ODBC and JDBC, which are standardized interfaces for connecting to databases.
What is %f in SQL?
%f is not a valid SQL keyword or operator. It may be a placeholder in a programming language such as Python, where it is used as a placeholder for a floating-point value. However, it is not part of SQL syntax.
What are the primary keys SQL?
Primary keys in SQL are columns that uniquely identify each row in a table. They are used to enforce data integrity and ensure that each record in the table can be uniquely identified.
What is == in SQL?
== is not a valid operator in SQL. In SQL, the equality operator is represented by =.
What is (+) in SQL?
(+) is a legacy syntax used in Oracle SQL to represent a left outer join.
Can a foreign key be null?
Yes, a foreign key can be null. This is known as a nullable foreign key.
What is the super key in SQL?
A super key in SQL is a set of columns that can be used to uniquely identify each row in a table. It may include more columns than the minimum required to form a primary key.
What is *= in SQL?
*= is not a valid operator in SQL. In SQL, the left outer join operator is represented by LEFT OUTER JOIN or LEFT JOIN.
What are indexes in SQL?
Indexes in SQL are data structures used to improve the performance of database queries. They allow data to be retrieved more quickly by providing a way to look up records based on the values in one or more columns.
What is a unique key in SQL?
A unique key in SQL is a column or set of columns that has a unique value for every row in a table. It is similar to a primary key, but it is not necessarily used to identify each row in the table.
What is role schema?
Role schema is a type of database schema that organizes objects based on the roles they play in the database. This approach can make it easier to manage complex databases with many different types of objects.
Explain the SELECT statement and its syntax?
The SELECT statement is the most commonly used SQL command. It is used to retrieve data from a database. The syntax for the SELECT statement is as follows:
SELECT column1, column2, …
FROM table_name;
The columns you want to retrieve are listed after the SELECT keyword, separated by commas. The table from which you want to retrieve data is listed after the FROM keyword.
What is a WHERE clause?
The WHERE clause is used to filter the results of a SELECT statement. It specifies a condition that must be met in order for a row to be included in the result set. The syntax for the WHERE clause is as follows:
SELECT column1, column2, …
FROM table_name
WHERE condition;
The condition can be any valid expression that evaluates to true or false.
How do you sort the result set in ascending or descending order?
To sort the result set in ascending order, use the ORDER BY clause with the ASC keyword. To sort the result set in descending order, use the ORDER BY clause with the DESC keyword. The syntax is as follows:
SELECT column1, column2, …
FROM table_name
ORDER BY column1 ASC/DESC;
You can sort by one or more columns, separated by commas.
Explain the difference between DISTINCT and GROUP B?
DISTINCT and GROUP BY are used to eliminate duplicates in a result set. DISTINCT is used to eliminate duplicates across all columns in the result set. GROUP BY is used to group the result set by one or more columns and then eliminate duplicates within each group. The syntax for DISTINCT is as follows:
SELECT DISTINCT column1, column2, …
FROM table_name;
The syntax for GROUP BY is as follows:
SELECT column1, column2, …
FROM table_name
GROUP BY column1, column2, …;
The columns listed after the GROUP BY keyword are the columns to group by. The SELECT statement can also include aggregate functions such as SUM, COUNT, AVG, etc., which are calculated within each group.
Can foreign key have duplicates?
No, a foreign key cannot have duplicates. Each value in the foreign key column must reference a unique value in the primary key column of the related table.
Can a table have multiple primary keys?
No, a table can only have one primary key. However, a primary key can consist of multiple columns.
How do I find duplicate rows in SQL?
To find duplicate rows in SQL, you can use the GROUP BY clause with the HAVING keyword to group the rows by the columns that may contain duplicates. You can then use the COUNT function to count the number of rows in each group, and filter the result set to only include groups with a count greater than 1.
What is a query in SQL?
A query in SQL is a request for data from a database. It typically consists of a SELECT statement that specifies the columns to retrieve and the table(s) to retrieve them from, along with any filtering or sorting criteria.
What are triggers in SQL?
Triggers in SQL are special types of stored procedures that are automatically executed in response to certain database events, such as INSERT, UPDATE, or DELETE statements. They can be used to enforce business rules, perform complex calculations, or update related data in other tables.
How many SQL keys are there?
There are several types of keys in SQL, including:
- Primary Key: A primary key is a column or set of columns in a table that uniquely identifies each row in the table. It cannot contain null values and can only have one primary key per table.
- Foreign Key: A foreign key is a column or set of columns in one table that refers to the primary key of another table. It is used to establish a relationship between two tables.
- Unique Key: A unique key is a column or set of columns in a table that ensures that each value in the column is unique. Unlike a primary key, a unique key can contain null values, and a table can have multiple unique keys.
- Candidate Key: A candidate key is a column or set of columns in a table that can be used as a primary key. It must be unique and cannot contain null values.
- Composite Key: A composite key is a primary key that consists of two or more columns.
- Super Key: A super key is a set of columns in a table that uniquely identifies each row in the table. It can contain extra columns that are not strictly necessary to ensure uniqueness.
- Alternate Key: An alternate key is a candidate key that is not selected as the primary key.
- These are the main types of keys in SQL, although some databases may have additional types or variations of these keys.
What is an INNER JOIN in SQL?
An INNER JOIN in SQL is a type of join that combines rows from two or more tables based on a matching condition in both tables. Only the rows that satisfy the matching condition are included in the result set.
How would you query data with conditions spanning multiple tables?
To query data with conditions spanning multiple tables in SQL, you can use the JOIN keyword to combine the tables, and the WHERE clause to specify the conditions that must be met for the rows to be included in the result set.
What is the purpose of the GROUP BY clause?
The GROUP BY clause in SQL is used to group the result set by one or more columns. It is typically used with aggregate functions, such as COUNT, SUM, AVG, and MAX, to calculate summary statistics for each group.
What does it mean to “normalize” a database?
Normalizing a database in SQL involves organizing the data into multiple tables with well-defined relationships between them. This can improve data integrity, reduce redundancy, and make the database easier to maintain and update.
Explain the difference between NULL and NOT NULL constraints in SQL.
NULL and NOT NULL are constraints in SQL that determine whether a column can contain null values. A NULL constraint allows a column to contain null values, while a NOT NULL constraint requires the column to always contain a non-null value.
What are some differences between SQL and NoSQL databases?
SQL databases are typically structured and use a relational data model, while NoSQL databases are typically unstructured or semi-structured and use a document, key-value, or graph data model. SQL databases are better suited for complex transactions and structured data, while NoSQL databases are better suited for large-scale data storage and flexible data models.
Can you share some projects in which you used SQL?
As a fresher, I do not have any professional experience working on projects that involve SQL. However, I have completed a few SQL courses during my academic studies, where I was required to complete projects that involved writing SQL queries and statements to retrieve and manipulate data.
For example, in one project, I had to design and create a database to manage student enrollment data for a university, and I used SQL statements to create tables, insert data, and retrieve data based on various criteria such as student ID, enrollment date, and program of study. Another project involved analyzing sales data for a retail company, and I used SQL queries to the group and aggregated the data to identify trends and patterns in sales performance.
Advantages of Using SQL
- Powerful Data Manipulation: Complex queries with aggregations, joins, and analytics.
- Mature Ecosystem: Widely supported in open-source (PostgreSQL, MySQL) and commercial (Oracle, SQL Server) systems.
- Scalability & Performance: Indexes, partitioning, and query planners optimize large datasets.
- Interoperability: Connects easily to BI tools, programming languages, and ORMs.
- Standardized Skillset: Learning SQL transfers directly to many database platforms.
Important Features of SQL
- Declarative Syntax
You describe what data you want, not how to get it. - ANSI & ISO Standards
Core SQL is standardized, making skills portable across databases. - ACID Transactions
Ensures data integrity through Atomicity, Consistency, Isolation, Durability. - Rich Data Types & Functions
Support for numeric, text, date/time, JSON, XML, spatial types, and built-in functions. - Advanced Querying
Joins, subqueries, views, window functions, CTEs (including recursive). - Security & Permissions
Role-based access control, encryption at rest/in transit, auditing features.
Bonus SQL interview Questions to practice
- How can you create a new table using SQL?
- How can you insert data into a table using SQL?
- How can you retrieve data from a table using SQL?
- How can you update data in a table using SQL?
- How can you delete data from a table using SQL?
- What are some of the most commonly used SQL commands?
Conclusion
SQL is a powerful language used to manage relational databases. It provides the ability to query, update, insert, and delete data from tables. When correctly implemented, it can improve data integrity, reduce redundancy and make the database easier to maintain and update. SQL has many commands and features that are important for any developer or DBA to be familiar with. By practicing the SQL interview questions in this article, you can become more prepared for job interviews and work confidently with relational databases. Good luck!