8 SQL Interview Questions (with Answers) for 2023
Job interviews can be a particularly stressful process for both individuals who are job searching for the first time and those who are more experienced.
This is equally true for a data scientist or developer who is required to demonstrate their technical knowledge in tools and programming languages such as SQL.
Therefore, preparation and organization before each stage of the interview in these competitive fields are the keys to better performance that will bring the candidate one step closer to success and employment.
So, if you are in the process of job hunting as a Data Scientist or Developer and want to know what potential interview questions you may be asked in order to be adequately prepared, you can read the following guide.
In today's article, we have gathered the 8 most common interview questions for SQL with their corresponding answers.
Let's get started.
SQL Interview Questions
SQL interview questions can vary in difficulty depending on the level of knowledge and position within a company, whether it's an entry-level position or a more advanced one.
In this article, we have compiled the following questions for someone who is taking their first steps in SQL.
Let's start with the first question.
Question #1: Where is SQL Applied?
A highly probable question that you may be asked about SQL during your interview is about where SQL is applied.
Let's see how you could answer this.
SQL (Structured Query Language) is a programming language used for interacting with relational database management systems (RDBMS).
SQL allows querying a database in various ways.
It can be used in any company that stores and analyzes data, facilitating easy searching and management of this data.
SQL has various applications, including creating and updating tables in a database, retrieving and summarizing important information from one or more tables.
Now that we've covered some basic aspects of SQL, let's move on to some more technical details.
Question #2: What Is the Difference Between a Primary Key and a Unique Key?
A primary key is a column or a set of columns that uniquely identifies each row in a table.
A table can have only one primary key.
On the other hand, a unique key is also a column or a set of columns that uniquely identifies each row in a table; however, a table can have more than one unique key.
The primary key and the unique key share a common element, but there are some individual differences:
- A primary key cannot have NULL values, while a unique key can have one NULL value.
- A primary key creates a clustered index on the column, while a unique key creates a non-clustered index by default.
- A primary key can be referenced by a foreign key from another table, while a unique key cannot be referenced by a foreign key.
Note: A foreign key is one or more columns of a table on which a constraint is enforced to link this column(s) with the primary key of another table. The purpose of these keys is to keep various tables within a database connected.
Question #3: What Does the Term JOIN Mean, and What Types Are There?
Another question that can be asked during your SQL interview is to discuss the term JOIN.
The term JOIN is used to combine and retrieve records from two or more tables. There are several types of joins:
INNER JOIN: An INNER JOIN returns only the rows that match the join condition in both (or all) tables.
LEFT (OUTER) JOIN: A LEFT (OUTER) JOIN returns all rows from the left table and the corresponding rows from the right table or a NULL value if there is no match.
RIGHT (OUTER) JOIN: A RIGHT (OUTER) JOIN returns all rows from the right table and the corresponding rows from the left table.
FULL (OUTER) JOIN: A FULL (OUTER) JOIN returns all rows from both tables, with zero values for unmatched rows.
Question #4: What Is the Difference Between UNION and UNION ALL?
UNION and UNION ALL are two SQL operators that combine the result sets of two or more queries into a single result set. The difference between them lies in how they handle duplicate rows.
Specifically, UNION removes duplicate rows from the final result set, while UNION ALL retains duplicate rows in the final result set.
Question #5: What Is the Difference Between the GROUP BY and ORDER BY Clauses?
GROUP BY and ORDER BY are two SQL clauses used for organizing data in different ways.
The GROUP BY clause is used to group data into subsets based on one or more columns or expressions.
The ORDER BY clause is used to sort data in ascending or descending order based on one or more columns or expressions.
Question #6: What Is the Difference Between the WHERE and HAVING Clauses?
The WHERE and HAVING clauses in SQL are used to filter data based on certain conditions. They differ in when they are applied and what types of conditions they can use.
The WHERE clause is applied before data is grouped using the GROUP BY clause (as mentioned earlier) and can only use conditions that involve columns, constants, operators, and functions.
In contrast, the HAVING clause is applied after data is grouped using the GROUP BY clause and can use conditions that involve columns, constants, operators, functions, and aggregate functions.
Question #7: What Is a Subquery in SQL?
A subquery is defined as a query that is nested within another query and can be used to return a single value, a list of values, or a table as part of the main query.
Furthermore, it can be used in various clauses of the main query, such as SELECT, FROM, WHERE, HAVING, and ORDER BY.
Let's assume that we want to find the average order amount for each customer.
We can use a subquery to calculate the average order amount for each customer_id in the orders table and then join it with the customers table to display the customer's name and the average order amount.
The query could be formulated as follows:
Question #8: What Is an Index in SQL?
In closing, a very common SQL question pertains to indexing and how it is used. An illustrative answer is as follows:
An index is a data structure that stores a subset of columns and rows from a table in a sorted order.
An index can be used to speed up queries involving operations such as filtering, sorting, joining, or aggregating data based on those columns.
To create an index, you can use the CREATE INDEX statement, followed by the name, table, and columns of the index.
For the use of an index, the database server will automatically select the best index to use based on the specific query.
For example, suppose we want to create an index on the customer_id column of the orders table to accelerate queries that filter by customer_id.
It could be formulated like this:
CREATE INDEX idx_orders_customer ON orders (customer_id);
So, we've covered 8 important SQL interview questions along with the corresponding answers.
This way, a data scientist or developer can have a better understanding of potential questions they might be asked and be better prepared for an interview.
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