SQL seems to be one of the top rising programming languages (popularity-wise), even though it isn’t technically a programming language in the first place! How does this happen and how is it even possible? Well, that’s exactly what I’m going to tell you - together with the most essential SQL interview questions to help you land that dream job of yours!
In this tutorial, we’re going to cover everything between the basic and advanced info that a potential employer might ask you. After that, we’ll talk about a few general things to keep in mind before, during and after your job interview.
Table of Contents
- 1. The Fundamentals of SQL
- 1.1. Question 1: What is SQL?
- 1.2. Question 2: What are the differences between the ‘delete’ and ‘truncate’ commands?
- 1.3. Question 3: What is a SQL Server?
- 1.4. Question 4: What is MySQL?
- 1.5. Question 5: What is a ‘table’ and a ‘field’?
- 1.6. Question 6: What is a ‘join’?
- 1.7. Question 7: What is ‘view’?
- 1.8. Question 8: What is a ‘constraint’?
- 1.9. Question 9: How would you display the current time and date with SQL queries?
- 1.10. Question 10: What is ‘denormalization’?
- 1.11. Question 11: What’s a ‘query’?
- 1.12. Question 12: Explain what are the different subsets.
- 1.13. Question 13: Do you know what a Primary Key is?
- 1.14. Question 14: Explain what a Unique Key is.
- 1.15. Question 15: Can you tell what data integrity is?
- 2. Advanced SQL Questions
- 2.1. Question 1: Explain ‘index’.
- 2.2. Question 2: What to do when you forget your root password?
- 2.3. Question 3: Can you say that NULL values are equal to a zero?
- 2.4. Question 4: What should you do if the data disk is overloaded?
- 2.5. Question 5: What’s ‘auto-increment’?
- 2.6. Question 6: Name the components that make up the most basic MySQL architecture.
- 2.7. Question 7: Create an empty table from an existing one.
- 2.8. Question 8: How would you get the current SQL version?
- 2.9. Question 9: How would you fetch alternative odd records from the table?
- 2.10. Question 10: Please select a unique record from the table.
- 2.11. Question 11: Explain the variables of SQL.
- 2.12. Question 12: What’s a ‘datawarehouse’?
- 2.13. Question 13: What is the main usage for a ‘recursive stored procedure’?
- 2.14. Question 14: How would you retrieve the first 3 characters from a character string?
- 2.15. Question 15: Please retrieve common records from two tables.
- 3. General Tips
- 4. Conclusions
The Fundamentals of SQL
As I’ve said, let’s take it from the top and start at one of the most commonly asked interview questions.
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Question 1: What is SQL?
At the very beginning, I’ve mentioned that SQL isn’t a programming language at all. So what is it?
SQL stands for Structured Query Language. It is primarily a database management language. On its own, it isn’t a programming language per se, but its standard may extend it to being a fully legit coding language - that why most people simply refer to it as a “programming language”.
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Question 2: What are the differences between the ‘delete’ and ‘truncate’ commands?
This is one of the better examples of basic SQL interview questions.
The main difference is that the “delete” command deletes one row from the table, while “truncate” deletes all of them.
Another key difference is that when you use “delete”, if something goes wrong or not according to plan, you can perform a data rollback - this can’t be done after the “truncate” command is issued. Also, truncate is quite faster.
Question 3: What is a SQL Server?
You are bound to get some SQL Server interview questions during your job application, so I’ve added some of the main ones in this tutorial.
SQL Server is a Database Management System (DBMS). It gathers and analyzes data, updates and administers the database.
Question 4: What is MySQL?
As with the SQL Server, questions about MySQL are also common thing during job interviews involving SQL.
MySQL is an open-source database management system. It provides a wide range of available functions and interfaces, has cross-platform support and possesses nifty tools to help you get to work in no time.
Question 5: What is a ‘table’ and a ‘field’?
Don't get tricked with these SQL interview questions. A “table“ is simply organized data made into separate rows and columns. A “field” is the number of columns in a table.
Question 6: What is a ‘join’?
The “join” command combines rows from different tables. The way that these rows are combined depends on the related column that joins them. With this command, you can merge two tables into one, or simply transfer data from one table to another.
Question 7: What is ‘view’?
A “view” is simply a virtual representation of a "table". More so, views can have more than one table’s data represented and combined - it’s very situation and relationship-dependant.
Question 8: What is a ‘constraint’?
“Constraints” are commands that are used to underline and specify the amount of a specific type of data used in a single table.
Question 9: How would you display the current time and date with SQL queries?
“How to” SQL interview questions are also common and normal, so don’t be surprised if you get one or two during your interview.
SQL has an inbuilt command GetDate() that retrieves and showcases the current time and day.
Question 10: What is ‘denormalization’?
Denormalization is a process when you access the database from the higher forms towards the lower forms. This process is performed to increase database performance.
Question 11: What’s a ‘query’?
It might sound like one of the more self-explanatory questions, and if that’s your initial thought after reading it - you’re right.
As the term itself implies, a query is simply a question. This question is sent to the database via a specific code to retrieve some sort of information from it.
Question 12: Explain what are the different subsets.
There are three subsets to know in SQL interview questions:
- Data Definition Language - it allows performing such operations as Create, Deletes, Alters objects.
- Data Manipulation Language - it helps to insert, update, retrieve data from a database or delete it.
- Data Control Language - it allows controlling access to the database. It can grant or revoke access.
Question 13: Do you know what a Primary Key is?
The primary key is a column or a collection of columns that uniquely identify each row in the table.
Question 14: Explain what a Unique Key is.
Unique Key is what uniquely identifies a single row in the table as previously mentioned in Primary Key explanation.
Question 15: Can you tell what data integrity is?
It is the process that defines the accuracy of the data that is stored in the database. It also defines consistency and integrity constraints to apply business rules on the data whenever it is entered into an application or the database.
Advanced SQL Questions
Now that we’ve covered some of the more basic SQL interview questions, you should have a better understanding of what the beginning of your job interview will look like. After your potential employers see that you’ve cracked the interview questions and answers, however, they might one-up the odds and give you some advanced questions. This is why it’s better to come prepared - now we’ll move on to the more challenging questions.
Question 1: Explain ‘index’.
Indexes are methods of enhancing performance, namely - retrieving information from the database with higher speed and efficiency.
In total, there can be three types of indexes - clustered, non-clustered and unique.
Clustered indexes reorder the table and search for information with the use of key values, while non-clustered ones maintain the order of the table. Unique indexes forbid fields to have duplicating values.
A table can only have a single clustered index, but multiple non-clustered ones.
Although it might not seem like one of the more advanced SQL interview questions, the difficulty here lies in the “explain” part. This is honestly what makes a lot of these questions advanced - employers may require you to explain or expand on your given answer with a follow-up.
Question 2: What to do when you forget your root password?
If you forgot or lost your root password, start the database with the command of “skip-grants-table”. After you set the new password, restart the database in normal mode and enter the new password.
Question 3: Can you say that NULL values are equal to a zero?
No, no you can’t.
A “zero” has a numerical manner, while NULL signifies the absence of a character, whether it be because it is unknown or unavailable. Following the same logic, NULL isn’t the same as a blank space, either, for it is simply a character.
Question 4: What should you do if the data disk is overloaded?
You might encounter situations where you fill-up the data disk. You can't continue to do anything if it’s overloaded.
What you need to know during this SQL interview question is that in this situation is to apply what is known as a soft link. These links create a location where you can store your .frm and .idb files - and that is exactly what you should do. This will resolve the overload problem.
Question 5: What’s ‘auto-increment’?
“Autoincrement” is a command that allows you to generate and create a unique number for a new record within the table.
This keyword can be used on multiple platforms, but it might vary a little. Since you are most likely to get some SQL Server interview questions, let me just tell you - the replacement for the “auto increment” command in SQL Server is “identity”.
Question 6: Name the components that make up the most basic MySQL architecture.
Let’s try to mix in some MySQL interview questions, shall we?
The most basic MySQL is made up of three main components - query optimizer, connection manager and pluggable engines.
Question 7: Create an empty table from an existing one.
This would be an example of how to do it:
Select * into employeecopy from employee where 1=2
Question 8: How would you get the current SQL version?
It sounds like one of the more basic questions, but a lot of people might stumble on this due to a lack of prior research - it becomes like a trick question.
The way that you would get the most current SQL version is by issuing this command:
Question 9: How would you fetch alternative odd records from the table?
You would do it using this command:
Select employeeId from (Select rowno, employeetId from employee) where mod(rowno,2)=1
Question 10: Please select a unique record from the table.
The way that you would select unique records from a single table is by using the “distinct” command. Here’s an example:
Select DISTINCT employeeID from Employee
Question 11: Explain the variables of SQL.
During SQL interview questions you need to know that In SQL, there are two different variables - local and global.
Local variables are those which can only exist in one, single function. Opposite to that, global variables may be located through ought the entire program.
Question 12: What’s a ‘datawarehouse’?
A “datawarehouse” performs a similar function that a real warehouse does, only with data.
In a “datawarehouse”, data is stored from many different areas and sources. There, it is sorted out and prepared for usage.
Question 13: What is the main usage for a ‘recursive stored procedure’?
A recursive stored procedure is when a procedure acts upon itself up to a point when it reaches a set boundary.
The main benefit of this type of procedure is that it allows programmers to use the same code over and over again.
Question 14: How would you retrieve the first 3 characters from a character string?
There are quite a few ways to do this, but this is one of the more popular and easier ones:
Select SUBSTRING(EmployeeSurname,1,5) as employeesurname from employee
Question 15: Please retrieve common records from two tables.
You may do so by performing the below-located task:
Select employeeID from employee. INTERSECT Select EmployeeID from WorkShift
So, now you’re a bit more familiar with both the basic and more advanced SQL interview questions. Now that you know what to expect during your job interview, let’s move on to some more general tips for you to consider.
I’m not going to go over the very common tips like “get some sleep” or “wear a suit”.
In sports training, there is a specific method of training before a competition. As it grows closer and closer, you would decrease the volume of your training, but increase the intensity you would train for shorter periods, but tryhard like never before.
This is a great point of view that you can apply to your preparation for all of the SQL interview questions and answers. As the interview grows closer, you should spend less time preparing, but while you actually do, really concentrate on the material that you have to learn. That means no multitasking, no coffee breaks and so on. This will help you stress less and feel more confident as the day of the interview approaches.
Confidence is another thing. Try to be and look confident, but don’t overdo it so as not to come off as cocky. There’s a fine line between saying that you’re quite experienced in the field and telling your potential employer that you’re the best programmer that has ever lived or will live.
There are likely to be interviews where you get turned down until you find that dream job. You shouldn't look at it as a negative thing, though - the more job interviews you apply or get invited to, the more advanced and basic SQL interview questions you’ll hear - in turn, the more prepared and confident you’ll feel every time.
It is no secret that good programmers and more and more needed every single day. Potential employers are willing to pay hefty salaries for people that can prove that they’re efficient in the fields of programming and developing.
SQL is one of the most popular languages that are currently in demand. People appreciate it for the language’s simplicity and ability to be applied and used in many different areas. It’s quite easy to learn yet grants great rewards for people who are truly proficient in it.
There are a lot of different SQL-related questions that your potential employers might ask you. In this tutorial, we’ve covered just the main ones - even so, it’s just the tip of the iceberg. If you’re serious about getting that job, you should consider doing more in-depth research on the topic.
So, we’ve covered basic and advanced questions and I gave you examples of the most common SQL interview questions and answers. Furthermore, I also provided some general tips on how you can improve your chances of landing that job opportunity.