40 Top Most Asked SQL Interview Questions and Answers for Freshers

Are you looking for opportunities in data analysis, data science, or database administration? If yes, SQL is a crucial skill set that you should possess to explore more opportunities in the data domain. This way, you can enhance versatility and utility in diverse industries.

To help you prepare, we have compiled a list of the 40 top most-asked SQL interview questions and answers. So, strap in, prepare some notes, and get ready to crack your first SQL interview with flair colors. Let’s get started…

1. What is SQL?

SQL refers to Structured Query Language. It is a programming language for storing and processing information in a relational database management system. This significantly stores information in a tabular form, with rows and columns representing different data attributes and relationships between the data values. In short, SQL makes updating, inserting, creating, and deleting tables from a relational database a breeze.

2. What are SQL dialects? Give examples

SQL dialects are different versions of the SQL language, each adjusted to accommodate the unique characteristics of specific database systems. Although they all stem from standard SQL, these dialects differ considerably in syntax, functions, and capabilities. SQL dialects include both free and paid versions of SQL. They have the unique peculiarities of a relational database system. The most common SQL dialects are PostgreSQL, Oracle, SQL Server, and MySQL.

3. What is the primary key in SQL?

The primary key in SQL is a unique identifier of a row in the table. It is a single or a group of columns that accepts unique values for each row. It ensures data integrity by creating relationships between tables. Thus, whenever you press the INSERT INTO command to insert a new value in the table, the primary key should be unique. Primary keys must have UNIQUE values, and cannot have NULL values. A table can possess only one primary key, which may be composed of either a single column or multiple columns (fields).

4. What is a database?

A database is an organized form of dataset in a computer system. It can contain any type of data, including words, numbers, files, videos, pictures, schemas, tables, queries, etc. The most important thing is that you can access it in various ways. The Database Management System (DBS) plays a crucial role by allowing you to edit, retrieve, and store data.

5. Does SQL support the programming language?

Yes, SQL is indeed a programming language. However, it is wrong to say it can support general programming language. Instead, it supports a domain-specific programming language, meaning it solves problems within a domain. It does not intend to solve any problem outside of that specific domain.

6. What is a foreign key in SQL?

A FOREIGN KEY in SQL is a field (or collection of fields) in one table, that refers to the primary key in another table. The table created with the foreign key is known as a child table and the table with the primary key is called the referenced or parent table. It establishes a relationship between tables for data integrity. Also, it ensures data retrieval across tables if necessary.

7. How is RDBMS different from DBMS?

A relational database management system is a type of database that stores data in a row-based structure to connect data elements related to each other. However, DBMS (Database Management System) stores data elements in a file rather than a file. And yes, there is no relationship between different data elements.

8. Describe the difference between the CHAR and VARCHAR2 datatype in SQL

Both CHAR and VARCHAR2 store the character values. The main difference between CHAR and VARCHAR2 is that: CHAR is a constant datatype, meaning once initialized, you cannot change its value in the process. CHAR stores only fixed-length character string data types. On the other hand, VARCHAR2 is a variable datatype. It stores variable-length string with specified upper limit of length. You cannot change its value multiple times during the execution time.

9. What are tables and fields in Structured Query Language (SQL)?

A table is the most basic unit of a database. It is an organized form of storing data into rows and columns in the SQL where each column represents fields and each row represents records. A single table holds records, and each record is stored in a row of the table. Tables are the most used type of database objects or structures that hold or reference data in a relational database.

10. What is Self-Join?

Self-join is a type of join operation in which you join the table with itself. This way, it allows you to compare data within a table by treating different rows as if they belonged to separate tables.

11. What is the meaning of a subquery?

A subquery refers to a query that is inside the query statement. Also known as nested query, you can use it in SELECT, INSERT, UPDATE, or DELETE statements. The user can also use it in other subqueries. Essentially, they are used to find, specify, and return data for use inside the main query.

12. What is the meaning of a NULL value in the SQL?

A null value in SQL is used when the value is missing or unknown in the column. The key noteworthy point is that the null value is neither a zero value nor an empty string in the column.

13. What is a UNIQUE key in SQL?

A UNIQUE key in SQL is the set of columns or fields of a table that make records (rows) unique. It is like a primary key that ensures the uniqueness of the columns. However, the UNIQUE KEY accepts a null value but a primary key can’t. A table contains only one primary key but multiple unique keys.

14. What is Cross Join (Cartesian join)?

Cross Join, also known as Cartesian Join, is one of the types of joins that combines each row from one table with each row from another table. It intends to produce a result table or the Cartesian product of the rows. The time-taken to create Cartesian product or join depends on the size of the table. For instance, if the tables are large in size, the process of the cross-join takes a longer time.

15. What is the meaning of data definition language?

A Data Definition Language (DDL), as the name says, is a language that efficiently defines, manages, and modifies database structures. It allows the execution of SQL queries like CREATE, DROP, and ALTER. Therefore, DDL utilized in database systems constitutes a subset of SQL.

16. What is schema?

A schema is a blueprint of the data to show how data is organized in a relational database management system. It includes structural elements, such as tables, indexes, fields, database structure, etc.

17. What is a default constraint?

A default constraint is a database constraint that is used to fill the column with default values. The default value is evaluated when the row is created. A default constraint ensures data consistency in the table. It prevents the use of null values in columns where there should be default values.

18. How to update a table?

To update a table, one needs to use the following syntax:
UPDATE table_name
Set Col_1= value_1, column_2 = value_2
WHERE condition;

19. How can you select common records from two tables?

To select the common records from two tables, you need to use the following INTERSECT statement:
Select *FROM table_1
INTERSECT
Select *FROM table_1

20. What is the DISTINCT statement?

The SQL DISTINCT statement is used to retrieve unique values from a specified column or set of columns in a database table. The SQL DISTINCT statement helps users to remove duplicate records, ensuring that only distinct, non-repeated values are returned.

21. What is difference between SQL and Star Schema?

SQL, or Structured Query Language, is a programming language. It is used to manage and manipulate relational databases. It allows users to perform operations like querying data, updating records, and creating or deleting tables in a database. SQL queries include SELECT, INSERT, UPDATE, and DELETE. It helps users interact with the data stored in databases like MySQL, PostgreSQL, or Oracle. Star Schema, on the other hand is a type of database schema used in data warehousing to organize data for query efficiency. Data is divided into fact tables (which store quantitative data) and dimension tables (which store descriptive attributes) in a star schema. Star schemas are used in OLAP (Online Analytical Processing) for fast querying in reporting and data analysis.

22. Describe the difference between SQL and NoSQL databases.

23. What is normalization in SQL?

Normalization in SQL is a systematic process to minimize data redundancy and maximize data integrity in the table. It helps to organize the data into a database and eliminates the duplicate data from the relational tables. Also, it does not accept undesirable elements, such as insertion, deletion, and update.

24. What is denormalization?

Denormalization refers to methods used for database optimization. It helps improve database queries by combining two or more tables. It involves adding redundant data to one or more tables Though it enhances data redundancy in tables, it reduces the number of joins required to improve the performance.

25. How should you use the WHERE Clause?

You should use the WHERE Clause to combine two data sources. This is done between those data sources that have matching data but different data fields. This is because it uses one field as a criterion for the other field with the use of the LIKE keyword.

26. What are indexes in SQL?

Indexes are special tables used by the database search engine. They speed up data retrieval from a table. Indexes prevent duplicate entries in columns or a combination of columns. Since they are typically used as a performance tool, they are highly used when the database grows in large size. This way, they ensure optimized query performance.

27. Explain GROUP BY in SQL

The GROUP BY in SQL partitions result rows into groups, based on their values in one or several columns. The GROUP By in SQL is used with aggregate functions like SUM, MIN, MAX, AVG, and COUNT to perform computation on data. The result of a query using a GROUP BY statement contains one row for each group. In short, you can say that it groups all the rows that contain the same values.

28. What is the difference between a DELETE command and a TRUNCATE command?

Look at the following table to understand the difference between a DELETE command and a TRUNCATE command:

29. What are the three types of operators available in SQL?

The following three types of operators are available in SQL:

• Arithmetic operators
• Logical operators
• Comparison operators

30. What are some common scalar functions?

Some common scalar functions are:

• CONCAT concentrates two or more-character strings
• MID extracts substrings from a group of string values
• NOW returns the date and time
• LEN estimates the total length of a column
• RAND calculates the collection of random numbers.

31. What do you mean by aggregate functions?

An aggregate function is a mathematical computation. These functions are typically used to derive descriptive statistics. It takes a set of values and returns a single value representing the significance of the data. The GROUP BY Clause of the SELECT Statement often accompanies it. In a nutshell, it summarizes a large size database.

32. What are character manipulation functions?

Character manipulation functions to edit, update, delete, or change character strings. The most common types of character strings are CONCAT, SUBSTR, INSTR, TRIM, REPLACE, RPAD, and LPAD.

33. What is a stored procedure?

A stored procedure is a prepared SQL code that can be saved so the code can be reused over and over again. A stored procedure performs one or more DML functions on databases. In short, it can

• Accept input parameters and perform operations based on the input that is available in the form of parameters.
• Return multiple values to indicate success or failure.

34. What is an ALIAS Command?

An ALIAS Command is a temporary name, assigned to a column or table inside a query. This way, it makes column and table names more concise and easier to read. Additionally, it is useful to resolve naming conflicts when you combine two or more tables.

35. What is a T-SQL?

Transact-SQL (T-SQL) is a programming language extension. Its major advantage is that it enhances the capabilities of the Structures Query Language (SQL). Very often, users use T-SQL to create, edit, and retrieve data in Microsoft SQL server databases.

36. How to duplicate a table in SQL?

Sometimes we need to create a copy of the existing table in SQL. Here are the steps that you should follow:

• Ensure you are connected to the database in which you want to duplicate a table
• Select the database in Object Explorer
• Go to the Object Explorer and right-click the table that you need to copy
• Then, select the option Design
• Then, select the Columns in the existing table
• Go to the Edit menu and select the option Copy
• Switch back to the new table and then, choose the first row
• Again, go to the Edit menu to select Paste
• Then, navigate to the File menu and select Save table name
• Finally, input a name for a new table in the Choose Name dialog box
• Select OK to confirm.

37. How can you optimize SQL queries?

Some common strategies to optimize SQL queries are:

• Use indexes
• Use of the WHERE clause
• Avoid SELECT Queries
• Minimize unnecessary data retrieval or data redundancy
• Incorporation of data types and layouts
• Use EXIST queries instead of COUNT queries
• Analyze query performance

38. What is the difference between primary key and unique constraints?

The basic difference between primary key and unique constraints is that there is one primary key in a table but there can be multiple unique constraints in a table. The primary key efficiently creates a clustered index automatically but the unique constraint cannot generate it.

39. Define WITH Clause in SQL

WITH clause in SQL, also known as a Common Table Expression (CTE), is used to create temporary results that the user can use in other queries. These can be used in SELECT, INSERT, UPDATE, or DELETE statement. It makes complex queries more readable by breaking them into smaller, named, and reusable subqueries.

40. What are the various types of relationships in SQL?

The different types of relationships in SQL are

• One-to-one relationship
• One-to-many relationship
• Many-to-one relationship
• Self-referencing relationship

41. What is a composite primary key?

A composite primary key is a database key. It is made up of two or more columns, and the combination of values in those columns must be unique for each row in the table. Columns in a table work together to uniquely identify each row.

The Bottom Line

So, here we end out list of the 40 top most asked SQL interview questions and answers for freshers. Hopefully, this article has been enlightening for you and helped you have deeper understanding of the questions that you’ll have to be ready with. These questions are frequently asked in interviews. So, it is advisable to prepare these questions along with answers to be sure of cracking the interview with flying colors. In addition to preparing these questions, make sure to be calm and composed. Staying calm can help you answer confidently and with energy, which enhances your chances of success. Remember that confidence is an ultimate strategy to nail all kinds of interviews. Happy Preparing… 😊 😊

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