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Top 20 Linux and SQL Interview Questions for Java and IT Professionals

Hello guys, If you have worked as a software developer or Java programmer then you know that SQL, Linux, and Networking fundamentals are essential skills for any Java developer, especially for server-side Java programmers. It actually doesn't matter whether you are applying for the job as a Java developer or C++ developer, Python developer or Ruby programmer, SQL and UNIX always have some role to play in your career. It's even essential for people who have less to do with programming like application support guys, business analysts, project managers, and subject matter experts.  Hence, it's imperative for any programmer or IT professional to prepare both SQL and UNIX well before going for any job interview.

This becomes even more important if you are going for a support engineer role instead of for a development role because then the importance of SQL, Database, and Linux skills increases as compared to the programming language you know.

Since the main job of support professionals is to look after the production environment, which often runs on Linux, it's crucial for them to have a good knowledge of both the Linux operating system and the command line.

They should know what they are doing because incorrect use of grep or rm command can bring down the whole production system. That's why they are extensively questioned and tested in their UNIX skills.

Same is true for SQL because most of the real-world system uses a relational database for backend like Oracle, MySQL or Microsoft SQL Server and support professionals often need to run queries, study stored procedure or write procedures to generate reports.

Just like the UNIX command, a careless SQL query also has the potential to bring down your production system to halt, and you should always do due diligence before running any query against a live environment like testing them in UAT or QA environment before running them in Production.

This single thing has saved my life a couple of times, and I have jolted a couple of points you should take care of while running queries on production or live database; you can find those tips here.

Because of these reasons, SQL and UNIX become essential skills for any software support professional. If you think you lack SQL and Linux skills, then I suggest you first go through these free SQL courses as well as these free Linux courses before appearing for any interviews. These courses are great to fill any gaps you have in your understanding.

20 Linux and SQL Interview Questions for Software Engineers and Java Developers 

This list contains some of the questions which I think very important from a Java support perspective, and many of them are actually asked in real interviews. If you are going for any Java support role, you can expect some SQL and Linux questions along with Core Java questions.

I encourage you to go through this list to get an idea of what to expect in a real Java support interview.

Btw, this is the second part of my list of interview questions for Java IT support professionals. In the first part, I have shared 30 essential Java and JEE questions, and in this part,  I'll share 17 SQL and UNIX questions from various Java Support Interviews.

Anyway, here are some of the frequently asked SQL and UNIX questions from Programming Job interviews:

1. How do you find the heap dump files in Linux?  (answer)
The heap dump files are generally in the form of java_pid.hprof, so you can easily search them using the find command in Linux as shown below:
$ find . -name *.hprof -print

2. How do you find the files currently used by your Java process?  (answer)
You can use the lsof command to find all the files, including sockets, by your Java process in Linux.  The lsof command stands for list open files, and since everything in UNIX is a file, it can list both normal files as well as open socket connection by any process.
$ lsof -c java
will list all files opened by the java command, and,
$ lsof -p 22342
will list all files opened by process id 22342.

For a more in-depth discussion on using the lsof command and its various open, I suggest you go through the Linux Command Line Basics course on Udemy. It covers all basic Linux commands with real-world examples.

SQL and Linux Interview Questions with answers

3. How do you find the ports currently used by a Java process?  (answer)
You can use the netstat or lsof command to find all the ports currently used by a Java process, but for that, you need the process id of your Java process. You can find the PID using the ps and grep command, as shown below:
$ ps -ef | grep java | grep myprocess

Once you find the PID of your Java process, you can use the netstat command to find the ports currently used in this Java process as shown in the following example:
$ netstat -nap | grep PID

If netstat is not available on your Linux machine you can also use the lsof command to find all the ports used by a Java process as shown below:
$ lsof -i

If you want to learn more about lsof command, a "Swiss Army Knife for programmers," you can check out my earlier post, 10 ways to use the lsof command in Linux.

4. What will happen when a TCP/IP client stops consuming messages from Server, but it's connected? Assume the Server is continuously sending messages. (answer)
The message will be stored on sending buffer at the sender end because it will stop receiving the ack from the receiver. If the server keep continues to push messages, the sending buffer will overflow, throw an error to stop the server.

Difference between TCP and UDP

5. What guarantees are provided by TCP/IP?   (answer)
The TCP/IP provides the guaranteed delivery, but apart from that it also includes an ordering guarantee, i.e., messages will be delivered to the client in the same order they are sent from the sender.

This is one of the important reasons to consider TCP/IP for ordered messages, e.g., orders and trades in the financial world.

Btw, if you want to learn more about TCP protocol, you can see this here to find some good books to learn more about TCP/IP protocol from the application developer's perspective.

6. How do you find the PID of a process listening on port 8080? (answer)
If the process is running on a Linux operating system, then it's pretty straightforward; just use the netstat command to find the PID of the process, netstat -p shows the PID. Once you got the process id, you can use the ps command to see the process itself.

Btw, If you are not familiar with netstat, ps, or fundamental Linux commands, then I suggest you first go through Linux Command Line Interface (CLI) Fundamentals course on Pluralsight. It has helped me a lot to improve my Linux skills.

Linux Interview Questions for Java Programmers

7. What is multicasting?  (answer)
Multicasting is a networking concept that allows you to send the same messages to multiple clients without sending them to all, i.e. without broadcasting it. In the case of multicasting, there exists a multicast group, which is represented by a special multicasting IP address.

Any client who wants to receive the message sent to this IP address can join this group, and then the router will ensure that new messages will be delivered to all subscribers.

8. What is the difference between multicasting and broadcasting?  (answer)
Even though both are networking concepts to deliver messages to multiple clients, there is a subtle difference that exists between them. In the case of multicasting, messages are delivered to only machines that are part of the multicast group, while in the case of broadcasting, a message is delivered to all machines in the network.

9. What is a multicast group?  (answer)
A multicast group is a single machine or group of machines with a special multicast IP address, i.e. the IP address in the range of to 239. 255.255.255. The multicast group can only be used as the destination of a datagram, not as a source.

10. You received an alert that the file system is about to fill up, how do you find big files to remove?  (answer)
This is one of the common situations Java programmers face. In Linux, you can use the find command to search all big files. The find -size options list the file with their sizes.

11. Can you name 10 Linux commands you use daily?  (answer)
This is easy, right? You can name any Linux commands you use, but make sure you give a variety of commands, e.g. commands to search, networking commands, process management commands, CPU and memory profiling commands, etc.

Anyway, here is my list of some basic Linux commands:
  1. ls
  2. top
  3. ps
  4. find
  5. grep
  6. netstat
  7. df
  8. lsof
  9. kill
  10. sort
  11. chmod
  12. du
  13. curl

If you love books and want to learn more about basic UNIX commands, then please see  The Linux Command Line: A Complete Introduction, a complete guide to Linux commands.

Top 10 SQL and UNIX Interview Questions for Programmers

If you prefer online courses then check out the two courses I have to suggest to you before, I generally start with a course and then read a book for a more in-depth understanding.

Best SQL Interview Questions for IT Support Engineers and Java Programmers

Now that you have seen some of the frequently asked Linux/UNIX command-line questions from programming job interviews, let's discuss some SQL and Database interview questions. This list includes questions on SQL query and database fundamentals.

12. How do you find the second highest salary in the employee table? (answer)
There are many ways to find the second highest salary in the employee table, e.g. you can either use the co-related subquery, or you can use a ranking function like row_number() and rank().

The question can be even more tricky by adding duplicates, e.g. two employees with the same salary. In that case, you can choose between the ranking function to satisfy the requirement.

I have already written a detailed blog post about this topic, please see here for a couple of examples of finding the second highest salary in the employee table.

How do you find the second highest salary in the employee table?

13. How do you find the duplicate rows in a table?  (answer)
Again, like the previous question, there are several ways to find the duplicate rows in a table, e.g. you can use a ranking function like row_number which assign row number depending upon the value you ask, and for the same values as in the case of duplicate rows, the row number will be same.

You can also the group by clause to detect duplicates e.g.

select emp_name, count(*) from employee group by emp_name having count(*) > 1

This one of the classic ways to find the duplicate rows in a table, and it will work in almost all databases like Oracle, SQL Server, or MySQL.

14. How do you remove the duplicate rows from the table?  (answer)
This is the follow-up previous question and much tougher than the previous one, especially if you are not familiar with the ranking functions like row_number.

If you only know about the GROUP BY clause, then you will struggle to remove duplicate rows because if you remove by emp_id, both rows will be removed. Here you must use the row_number() function, the duplicate rows will have row number > 1 like

select emp_name, row_number() over (order by emp_name desc) row_number 
from Employee

This will print something like below.

John        1
Heroku    1
David      1
David      2

Here second David is duplicate, and you can easily remove it by giving conditions like delete from table where row_number> 1.

However, if you are not familiar with window functions like rank and row_number(), I suggest you go through an online course like Microsoft SQL for Beginners. This will not only help you to understand basic SQL commands but also provide some insight on how to use them effectively in your day-to-day work.

SQL Interview Questions for Programmers

15. How do you find all records modified between two dates in SQL?  (answer)
This is one of the tricky SQL questions. It looks simple, but when you go and write the SQL query, you will find that it's not working as expected. For example, many programmers will use the "between" SQL clause to find all the rows modified between two dates e.g.

select * from Orders where order_date between '20160901' and '20160930'

This will work well in most cases, but if there is an order where order_date is midnight of 1st October, then it will also be picked. You can be more precise by using the logical operator, as shown below:

select * from Orders where order_date >='20160901' and order_date < '20160930'

16. What is the difference between clustered and non-clustered indexes?  (answer)
This is one of the frequently asked database questions, the main difference between the two is that there can be only one clustered index on a table while you can have more than one non-clustered index. The clustered index also defines how the data is physically stored on the disk

. What is the difference between clustered and non-clustered indexes


17. What is the difference between count(field) and count(*) in SQL?  (answer)
The only difference between count(field) and count(*) is that the former doesn't count null values while the latter does. For example, if you have an emp_name column on your table and it contains 10 valid values and 3 null values, then count(emp_name) will return 10 while count(*) will return 13 rows.

That's all about SQL and UNIX questions for Programmers and Software Engineers. As I said, these two technologies are very, very important for any job, and good command of these two goes a long way to secure the job. They are also classic technologies, i.e. they don't obsolete every couple of years, and they will remain popular for several years to come.

I have interviewed a lot of support guys, particularly for Java applications, and I always have focused on their core Java and support skills along with SQL and UNIX, and fortunately, the people who have come out of those interviews have done very well on their job.

Other Interview Questions for Programmers
Along with these questions, you should also check the following interview questions shared before to do well on your support job interviews.

Thanks for reading this article so far. If you like these Linux, SQL, and Multicasting interview questions for Java programmers, then please share them with your friends and colleagues. If you have any questions, doubts or seeking an answer for something, please drop a comment, and I'll try to find an answer for you.

P. S. - If you want to learn Linux and SQL in-depth and looking for some awesome resources then you can also check this list of best Linux and SQL courses for programmers. It contains both free and paid courses to learn online. 

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