7 Advanced SQL Interview Questions (With Sample Answers)

Updated 19 March 2023

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Advanced SQL is a concept under SQL that professionals in many different fields use for maintaining or manipulating data. Many employers require potential candidates to have knowledge of advanced SQL while hiring for roles like software engineers or developers. Knowing some questions that you may answer during an advanced SQL interview and preparing answers for them can help you excel at the interview and increase your chance of getting the job. In this article, we list seven advanced SQL interview questions, provide their sample answers and list some tips to help you prepare better for your upcoming interview.

7 Advanced SQL Interview Questions With Sample Answers

Here are seven advanced SQL interview questions and their sample answers to help you prepare your own responses:

1. What is advanced SQL?

This may seem like a basic question but the answer to this can be tricky. Advanced SQL is not a separate programming language. Its definition may vary depending on who uses it and what advanced components of SQL they use. Interviewers may ask this question to test your knowledge of the advanced concepts of SQL. To answer this question, you can elaborate on the concept and state what advanced components you use while coding in SQL.

Sample answer: 'Advanced SQL can have different meanings for different users. It involves the use of advanced components of SQL while coding. Many may use SQL components like joins, aggregate functions, subqueries and set operations and call it advanced SQL. This is an incorrect definition of the concept. Advanced SQL includes the use of components like common table expressions and GROUP BY extensions. These components typically justify the use of advanced SQL'

2. What are indexes?

This is a basic question the interviewers may ask to understand your basic knowledge of SQL. They may find it important that you know the answer to this as indexes are an important component of SQL. In your response, define an index and also briefly explain the different types of indexes available.

Sample answer: 'An index is a set of keys related to a table that SQL stores separately. You can use an index to speed up the data retrieval from a specific table. It also enhances the performance and efficiency of the process. It contains various keys that you can use to retrieve specific sets of data from a table, just like an index that you find on the last page of a textbook.

The three main types of indexes are clustered indexes, non-clustered indexes and unique indexes. A clustered index reorganises a table and uses key values to search for data. A non-clustered index searches for data without reordering the table. A unique index checks for the uniqueness of the table and ensures that no row or columns contain duplicate values.'

3. Please define auto-increment and state its uses

This is another basic question the interviewers may ask. Auto-increment is a popular component of SQL and it may be vital for you to know its definition and uses. In your response to this question, briefly explain what it is and state its function.

Sample answer: 'Auto-increment is a basic, but essential component of SQL. It automatically generates a unique number when you insert a new record into a table. SQL typically uses the IDENTITY command to set up auto-increment. If you assign the number one to IDENTITY, it increments by one every time you enter a new record into the table.'

4. Explain variables in SQL

Interviewers may ask this question to gauge your practical knowledge of SQL. Variables are a common part of SQL coding. To answer this question accurately, explain what variables in SQL are and talk about the types of variables available and how to assign values to them.

Sample answer: 'A variable in SQL is a placeholder for a specific value. It serves as a unique identifier for the values you input into a database. As the data you enter into a database may vary, the identifier gets its name as a variable. There are two types of variables in SQL which are local variables and global variables. Local variables can only be present in a single function at any moment. Global variables can be present in multiple functions throughout the program.

You can define a variable in a program using the DECLARE statement. To assign a value to the variable, you can use the SET statement or the SELECT INTO statement.'

5. What are joins? Define some joins in SQL

This is another question that can help the interviewers determine your working knowledge of SQL. Programmers often use join statements to merge datasets in databases. You can define what join statements are and explain the different types of join statements available in SQL.

Sample answer: 'Join statements are useful in combining two or more rows that have at least one column between them with a common value. There are five main types of join statements which are inner join, left outer join, right outer join, full outer join and cross join. An inner join only returns data from every row of every table with the exact matching value. A left outer join returns all the values from the first table of the command and only the matching records from the second table.

A right outer join has the exact opposite function to the left outer join. It returns all the values from the second table of the command and only the matching records from the first table. A full outer join returns all the records from both tables if any of them have matching records. A cross join returns all possible combinations of the matching records in both tables.'

6. What are Common Table Expressions?

This is a question that requires advanced knowledge of SQL. Interviewers may ask this question to understand how well you understand this concept and if you know what function it has in SQL. In your response, accurately define Common Table Expressions and explain how they work.

Sample answer: 'Common Table Expressions or CTEs are basically temporary results that you can use with SELECT statements in SQL. CTEs became a part of the SQL server first in 2005. It allows developers to use temporary data structures or tables in a program. You can join the tables you create using a CTE with other CTES, other tables or with itself. A prominent advantage of using a CTE is that it can help developers create complex reports efficiently and quickly. It also allows complex calculations and enables developers to write complex queries without writing any subqueries.'

7. What are NULL values in SQL?

This is a practical but basic SQL question. Interviewers may ask this to gauge your understanding of NULL values in SQL or in any database. Explain what NULL values are and also clarify why they are not equal to zero.

Sample answer: 'A NULL value in SQL is simply a field that holds no value. If you add an optional field to a table, it is possible to leave that field empty while entering records into other fields. The table then saves that field with a NULL value. It is a misconception that NULL values are equal to zero but that is incorrect. A zero represents a numeric, so assigning that to a field would mean that the field has a value and is not empty.'

Tips For Advanced SQL Interview Questions

Here are some tips you can follow to perform well in your interview:

Read the job description. Reading and understanding the job description can help you comprehend the requirements of the job well. This may also allow you to prepare your answers in a way that communicates your suitability to the interviewers well.
Prepare answers in advance. Research some questions and prepare answers to them before the interview. You can also practice speaking those answers out loud to gain confidence.
Arrive early. Arrive at the venue of the interview at least 30 minutes early. This can help you get comfortable with the place and relax before the interview.