20 SQL Interview Questions You’ll Be Asked for a Developer Role
The fastest way to break into a developer position is through what I call the “Learn SQL First and Get Hired Strategy.” For many data analyst jobs out there, SQL might be all that is required for you to get your foot in the door in tech.
SQL deserves its own undivided attention because from my experience it’s the fastest ticket to getting hired in tech and it has the lowest barrier of entry.
Why all new developers should learn SQL first
Why you might ask? Let me explain. To get a job as a developer, you need to be proficient in something, and full-stack development requires proficiency in a variety of technologies.
This is why I always suggest learning SQL first! It doesn’t take long if you give it the attention it deserves. If you focus your efforts on SQL, three months of learning is usually enough to be job-ready for a junior-level database developer role.
20 SQL interview questions
After having taken my Oracle SQL course, you will have learned SQL well, but how do you prepare for a SQL interview? Below are some of the most important SQL questions you should be able to answer.
1. Explain what SQL is: SQL stands for structured query language. SQL is the language that is used to create databases and tables, to update or retrieve data from the databases, and anything else involving organizing or using data.
2. Explain the difference between SQL and MySQL: SQL is a structured query language used to interact with relational databases. MySQL is just the name of a relational database management system. It is owned by Oracle the company.
3. Is SQL a programming language? No. SQL is a command language, not a programming language. SQL can be used to manipulate data, but it does not have the characteristics of a programming language such as conditional statements or loops.
4. Explain what data definition language (DDL) is: DDL statements are a subset of the SQL command set, used to make changes to a database. DDL statements are used to create, modify, and remove database objects such as tables, indexes, and users. (Examples of DDL statements include: CREATE, ALTER, DROP).
5. Explain what data manipulation language (DML) is: DML statements are a subset of the SQL command set, used to manage data within a table. (Examples of DML statements include: INSERT, SELECT, UPDATE).
6. Explain what data control language (DCL) is: DCL statements are a subset of the SQL command set and they allow you to manage the access and rights of a database. (Examples of DCL statements include: GRANT, REVOKE).
7. Explain what a query in SQL is: A query is a request to retrieve specific data from the database. The commands are written in SQL and typically start with using the SELECT keyword.
8. Explain the difference between a query and a statement: A statement gives commands to the database, whereas a query asks for something from the database.
9. Explain what a table in SQL is: Tables are database objects that refer to a collection of data that is organized in a row and column format.
10. Explain what a field in SQL is: Fields refer to the columns in a database. A specific field would be the intersection of a column and row.
11. Explain what the difference is between DELETE and TRUNCATE statements: A delete statement is used to delete specific records from a database table. A truncate statement is written as TRUNCATE TABLE, and it removes all rows from a table, but the table structure and its columns, constraints, indexes, and so on, remain.
12. Which one is faster: DELETE or TRUNCATE? In most cases TRUNCATE is faster. This is because DELETE first scans the table to generate a count of rows that were affected, and then deletes the rows one by one and records an entry in the database log for each deleted row. TRUNCATE TABLE simply clears all the rows without using additional system resources or indexes on the table.
13. Explain what a primary key is: A primary key is a column chosen to be represented as the unique identifier of each record in a table. It is enforced by a constraint on the table and the particular column can never have duplicates.
14. Give an example of when you have used a primary key: For a question like this, you’ll have to come up with an example that you have experimented with. For example, if you were working on a database containing vehicle information, a good primary key candidate for a vehicle table would be the vehicle identification number (VIN).
15. Explain what a foreign key is: A foreign key is a column that is configured to be used as a cross-reference between tables. This is managed by choosing one specific column to be the lookup for the cross-reference relationship, and then creating a foreign key constraint on that column.
16. Give an example of when you have used a foreign key: For a question like this, you’ll have to come up with an example that you have experimented with. For example, if you were working on a database containing vehicle information, in your vehicle table you could have a column called “Dealership_ID”, which is referenced from a separate table called Dealerships. The Dealership ID would be the foreign key inside the vehicle table.
17. What is a transaction in SQL: A transaction is an atomic unit of work that has a beginning and an end. It could be a single line of code that is executed or multiple lines of code. The key to understand is that a transaction ends when it is committed or rolled back, either explicitly with a COMMIT or ROLLBACK statement, or implicitly when a DDL statement is issued. For example, let’s say you updated data or deleted data in a table. If you don’t close the transaction by issuing a COMMIT command, the update will be temporary and will not be reflected in the database the next time you log in. You can undo the deletion or updating of data by issuing the ROLLBACK command. That will take you back to the previous COMMIT point.
18. Give an example of when you have had to use a transaction: Again, this requires you to give a story or an example. If you’ve taken my Complete Oracle SQL Certification course or my Master SQL for Data Science course, you should be able to come up with tons of examples for each of these questions. These are the top SQL courses on the market with mostly 5-star reviews.
19. Give examples of 4 different types of SQL Joins: INNER JOIN, FULL OUTER JOIN, LEFT JOIN, RIGHT JOIN. There are a handful of other joins too, such as NATURAL JOIN or EQUIJOIN, which I talk about in my Oracle SQL course. If you completed that course, coming up with examples should be pretty easy for you. Drawing out the relationships of different joins on a whiteboard or piece of paper will make it easier for you to explain the concepts.
20. Entity Relationship Diagrams: Designing database tables and Entity relational diagrams is another topic that will come up in an interview — for this, you’ll need to be able to design various table structures. I’ve created a youtube video explaining this topic thoroughly. Check it out here:
Additional SQL interview topics
In addition to the questions listed above, you’ll need to explain concepts on data normalization, indexes, and the different kinds of constraints that can be applied to various columns of a table. Several students that completed my SQL courses have been able to land jobs as SQL developers through practicing all of the assignments and training on the most important topics required to pass the interview and become a rock-solid SQL developer. Using SQL proficiently comes with a lot of practice and that is what I prioritize. In your interview, you’ll most likely be given a dataset to look at and asked to write various SQL statements, queries, and commands on a piece of paper or whiteboard. With some hard work and about three months of dedicated learning, you can become a proficient junior-level SQL developer and land an excellent job.
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