Guide to Effective SQL Job Interview Questions

If you’re an aspiring SQL developer looking to build a…

If you’re an aspiring SQL developer looking to build a career in the ever-growing SQL community, you’re at the right place! We have the perfect list of SQL job interview questions to immerse yourself in the knowledge pool and prepare for your forthcoming SQL interview.

This article lists the most practical SQL interview questions and the appropriate answers to help you prepare better.

Photo by Rubaitul Azad on Unsplash

How to Prepare for SQL Job Interview Questions

One of the most important aspects of any job interview is figuring out how to prepare for the job interview questions.

Give yourself plenty of time to prepare for the SQL job interview questions. Whether a fresher or a professional, it’s best to brush up on your SQL skills before an interview.

To prepare for an SQL job interview, you must understand the job. That means you have to have a good understanding of the job functions. Know the skills required for that job and the company’s expectations of the ideal candidate.

This will help you know what topics to prepare for in detail for the desired position.

List of Important Interview SQL Questions

We have compiled a thorough list of SQL job interview questions and answers that will be useful during an interview. Whether you’re a fresher or an experienced candidate, these questions will prepare you to enter various SQL-worthy job roles.

1. What Is Database?

A database is a structured collection of data that is digitally stored and accessible via a local or remote computer system. Set design and modeling techniques are occasionally used to build complex databases.

2. What Is DBMS?

DBMS refers to a Database Management System, a system software in charge of creating, retrieving, updating, and managing databases. It acts as an interface between the database and its users or application software. Thus, it ensures that our data is accurate, consistent, and simple to retrieve.

3. What Is RDBMS? How Is It Different From a DBMS?

RDBMS stands for Relational Database Management System. It stores and manages data in a tabular format, and relations can be established between the common fields of the tables.

The primary distinction between a DBMS and RDBMS is that RDBMS stores data as tables, whereas a DBMS stores data as a file.

4. How to Create a Table in SQL?

Creating a table in SQL is pretty straightforward. First, we will start with the CREATE TABLE keyword and then provide a name for the table. After that, we will list all the columns and their data types, enclosed in braces.

Here’s the command to create an SQL table:

A Visual Studio Code snippet for creating a table in SQL
A Visual Studio Code snippet for creating a table in SQL

5. What Is SQL?

SQL refers to Structured Query Language, the standard language for communicating with relational database management systems. It is used to handle organized data made up of entities and relations between those entities. This language performs various tasks like retrieval, updating, inserting, and deleting data from a database.

6. What Are Constraints in SQL?

Constraints in SQL specify the limit on the data type in a table. It can be used for both single and multiple fields in an SQL table. Constraints are specified when you alter or create the table statement.

Here are some typical constraints:

  • NOT NULL– Prevents the insertion of NULL values into columns.
  • DEFAULT– Automatically sets a default value if no value has been entered for the field.
  • CHECK– Verifies that each value meets a condition in a field.
  • INDEX– Indexes a field for faster record retrieval.
  • UNIQUE– Ensures that each value entered into the field is unique.
  • PRIMARY KEY– Uniquely recognizes each record in a table.
  • FOREIGN KEY– Confirms referential integrity for a record in another table.

7. What Is a Primary Key?

The Primary key constraint uniquely specifies a row in a table. This is a particular type of key that must contain unique values, and it cannot be NULL.

In SQL, a table is strictly limited to having a single primary key, which may consist of one or more fields (columns).

8. What Is a Unique Key?

A unique key is a kind of SQL constraint that limits the type of data you can enter into a table. A constraint may be applied at the column or table level. When a unique key constraint is given to a column, it means that the column cannot contain any duplicate entries. In other words, every record in this column needs to be unique.

Whenever we give a unique key constraint to a column, this means that the column cannot have any duplicate values present in it. In other words, all the records present in this column must be unique.

9. What Is a Foreign Key?

A foreign key ensures referential integrity by establishing a connection between data in two tables. A foreign key essentially refers to the primary key of another table. This foreign key prevents actions that would hamper the connection between the child and parent tables.

10. What Is a Join?

This keyword is used to query data from other tables depending on the connections between their fields. Keys are crucial when JOINs are utilized.

11. What Are the Types of Join and Explain Each?

There are various types of Joins in SQL, which are explained below:

  • (INNER) JOIN: Retrieves the records with matching values in two tables involved in the join.
  • LEFT (OUTER) JOIN: Returns all the rows from the left-hand side table and the matched rows from the right table.
  • RIGHT (OUTER) JOIN: Returns all the rows from the right-hand side table and the matched rows from the left table.
  • FULL (OUTER) JOIN: Returns all the matching records between the left and right tables.

12. What Is a Self-Join?

A self-join is a type of join that is used to link the rows of a table to itself based on some relation. This means that each row in a table is attached to itself and other rows of the same table in a self-join. You can use self-join to compare and combine rows from the same table in a database.

13. What Is Normalization?

Normalization is the process of minimizing redundancy and dependency by organizing fields and tables in a database. The primary goal of normalization is to add, delete or modify the fields in a single table. Normalization helps to decompose a complex table into a simpler and smaller form.

14. What Are All the Different Normalizations?

There are primarily three normal forms of a table which are:

  • First Normal Form (1NF): This should eliminate all the duplicate columns of the table. A relation in the first normal form means that each attribute value is atomic and cannot be broken down further.
  • Second Normal Form (2NF): It must fulfill all the conditions of the first normal form. There is no partial dependency between the non-key and key attributes.
  • Third Normal Form (3NF): This should meet all the requirements of the second normal form. There is no transitive reliance between non-key and key attributes through another non-key attribute.

15. What Is a Query?

A query is a request to one or multiple database tables for data or information. A Select and Action query are two main types of database queries.

16. What Is Subquery?

A subquery is nested inside another large query. The larger or outer query is called the main query, while the inner query is known as the subquery. The subquery is always executed first, and the output is sent to the main query.

17. What Is the Difference Between DELETE and TRUNCATE Commands?

The DELETE command removes rows from a table depending on the condition given in the WHERE clause. If no condition is given, it removes every row from the table. You can roll back data after using a DELETE command.

The TRUNCATE command is used to delete all the rows from a table. And you cannot roll back data after a truncate operation is executed.

18. What Are Local and Global Variables and Their Differences?

Local variables are those that can be utilized or already exist within a function. They cannot be referred to or used since the other functions are unaware of them. Every time that function is invoked, variables can be created.

Global variables can be used or accessed throughout the program. The same variables declared globally cannot be used in functions. Global variables cannot be created when that function is invoked.

19. What Is Collation?

Collation is a set of guidelines that specify how data can be sorted and compared. Character data is sorted according to the rules that establish the proper character order. It has the option to specify case sensitivity, character width, etc. These character data can be compared using ASCII values.

20. What Is CLAUSE?

An SQL clause is created to restrict the result set by providing a condition to the query. This typically extracts a few rows from the entire collection of records. A query with a limiting clause usually contains the WHERE or HAVING condition.

21. What Is Union, Minus, and Interact Commands?

The UNION operator combines the results of two tables while removing any duplicate entries from the tables.

The MINUS operator returns rows from the first query but not from the second. A result set will show the matching rows from the first and second queries and additional rows from the first query.

The INTERSECT operator combines and returns the results of both queries into a single row.

22. What Are Aggregate and Scalar Functions?

Aggregate functions assess mathematical calculations and return a single value as an output. These computations are made using a table’s columns. For example, max() and count(), for instance, are computed with regard to numeric.

Scalar functions also return a single value, but it is based on the input value. For example, UCASE() and NOW() are calculations that take string into account.

23. What is a recursive stored procedure?

A recursive stored procedure invokes itself until it encounters a boundary condition. Programmers can reuse the same set of code as often as they like with this recursive function or process.

24. How do you Fetch Alternate Records From a Table?

You can fetch records for both Odd and Even numbers.

Here’s the command to display Odd numbers:

A Visual Studio Code snippet for fetching odd numbers from a table
A Visual Studio Code snippet for fetching odd numbers from a table

Here’s the command to display Even numbers:

A Visual Studio Code snippet for fetching even numbers from a table
A Visual Studio Code snippet for fetching even numbers from a table

25. What Is the Command Used to Fetch the First 6 Characters of the String?

You can fetch the first six characters of a string in various ways, which are as follows:

Command 1:

The first command for fetching the first 6 characters of a string
The first command for fetching the first 6 characters of a string

Command 2:

The second command for fetching the first 6 characters of a string
The second command for fetching the first 6 characters of a string

Conclusion

Everyone wants to know what to expect when it comes to an interview for an SQL Database Administrator position. Sometimes interviewers ask fundamental SQL questions that we fail to recall, and you’ll be out of luck. You can better prepare yourself by practicing the SQL job interview questions in this guide to avoid messing up your interview. Good luck!

Guide to Effective SQL Job Interview Questions

Abir is a data analyst and researcher. Among her interests are artificial intelligence, machine learning, and natural language processing. As a humanitarian and educator, she actively supports women in tech and promotes diversity.

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