As the standard programming language for database management (and one of the most popular programming languages across the board), SQL is essential for any organization that wants to harness the power of data.
Businesses of all sizes and sectors depend on SQL developers to query data and perform data analysis; finding the right SQL specialist or team is a crucial step to becoming a data-driven company.
To make the right hire, it’s important to organize a multi-stage recruitment process that identifies candidates with the right SQL experience and work ethic for your organization. Interviews can probe into both areas, but as they’re very resource-intensive, we advise you to use them after an initial programming skills test, such as:
To get the most out of your interview process, consider using our SQL server interview questions and sample answers below.
We’ve divided the 50 questions into beginner, intermediate, and advanced categories, so that you can tailor the interview process to your business needs.
Beginner SQL interview questions are best used for entry-level SQL server roles, where the candidate is likely to have limited professional experience with SQL.
A Relational Database Management System (RDBMS) refers to the software used to store, manage, and query data. Data is stored in tables and can be linked to other datasets based on shared information, hence the name “relational”.
The key differences between Relational Database Management Systems (RDBMS) and Database Management Systems (DBMS) are:
An RDBMS stores data in a relational table with rows and columns, whereas a DBMS stores data as a file
An RDBMS provides access to multiple users (including client-server side interaction), whereas a DBMS only supports single users
Some of the most popular RDBMSs are:
Microsoft SQL Server
SQL is a programming language used to perform data-related tasks; every RDBMS uses SQL as its standard programming language. In these databases, SQL allows users to create tables, update data, make queries, and perform analytics.
SQL is the programming language used in an RDBMS, while MySQL is an example of an RDBMS. MySQL was one of the first open-source database systems on the market, and it is still fairly popular today.
The CREATE TABLE command is used to create a new table in an RDBMS. This command prompts users to fill in the table name, the column names, and the types of data. The same command can also be used to make copies of existing tables.
With SQL, the DATE data type is used to store data or time values in the database. The format for inserting dates can vary depending on the RDBMS, but generally it’s ‘YYYY-MM-DD’.
A query is a request for data or information from a database. There are two main types of SQL queries:
A select query is a query that groups data from a table for analytical purposes
An action query is a query that changes the contents of the database based on specified criteria
A subquery is a query that is embedded within another statement that requires multiple steps. The subquery provides the enclosing query with additional information needed to execute a task, such as when the completion of one query depends firstly on the results of another.
The process for performing a select query in SQL is as follows:
The SELECT statement is used to specify the columns you want to query
The FROM statement is used to specify the particular table holding the data
The WHERE statement is used to filter data based on specified conditions
There are several SQL statements for running an action query. Their purposes and procedures vary. Some of the important action statements include:
UPDATE modifies the values of fields in a table
DELETE removes records from a table
CREATE TABLE creates a new table
INSERT INTO adds records to a table
SQL constraints are a set of rules or conditions implemented on an RDBMS to specify what data can be inserted, updated, or deleted in its tables. This is done to maintain data integrity and ensure that the information stored in database tables is accurate.
The join clause combines columns with related values from two or more tables to create a new table. There are four main types of SQL join clause:
JOIN returns records with matching values in both tables
LEFT JOIN returns all records from the left table and matching records from the right table
RIGHT JOIN returns all records from the right table and matching records from the left table
FULL JOIN returns all records from both tables
An SQL index stores important parts of a database table to allow for a quick and efficient lookup. Rather than searching the entire database, users only have to consult the index during data retrieval. Indexes, therefore, help improve performance in an RDBMS.
A NULL value indicates the data is unknown. This is not the same as 0; NULL values mean no data is stored at all.
SQL commands are used to perform specific tasks within the database. There are five main types:
Data Definition Language (DDL) commands change the structure of the database
Data Manipulation Language (DML) commands modify data in database tables
Data Control Language (DCL) commands manage user access to the database
Transaction Control Language (TCL) commands manage transactions made by DML commands
Data Query Language (DQL) commands retrieve information from the database
Aliases are temporary names given to tables or columns for the duration of a particular SQL query. Their purpose is to reduce the amount of code required for that query, therefore saving time and effort.
You can use these intermediate SQL interview questions below for mid-level SQL roles, where candidates already have some professional experience using SQL.
Normalization is the process of dividing data into tables to remove redundant data and improve data integrity.
Denormalization is used to combine multiple tables in order to reduce the time required to perform queries.
The syntax for creating an index can vary depending on the RDBMS. In most systems, the CREATE INDEX statement is used to initiate the process. The user is then prompted to name the index and select columns that will make up the index.
Here are some of the most commonly used SQL constraints:
NOT NULL ensures a column cannot contain a NULL value
UNIQUE ensures all values in a column are different
DEFAULT provides a default value for a column when none is specified
INDEX creates an index for data retrieval purposes
CHECK checks values in a column against certain specified conditions
The key differences between clustered and non-clustered indexes are:
Clustered indexes define the physical order in which tables are stored and sort them accordingly, whereas non-clustered indexes create a logical order that doesn’t match the physical order of the rows on the disk
Clustered indexes sort data rows based on their key values, whereas non-clustered indexes use a structure separate from the data rows
There can be only one clustered index per table, whereas there can be multiple non-clustered indexes per table
There are three main types of SQL subqueries. These are:
Single-row subqueries, which return one row in results
Multi-row subqueries, which return two or more rows in results
Correlated subqueries, which return results according to outer queries
The cursor allows users to process data from a result set, one row at a time.
Cursors are an alternative to commands, which operate on all rows in a result set at the same time. Unlike commands, cursors can be used to update data on a row-by-row basis.
Collation refers to a set of rules or conditions that influence how data is stored and ordered. There are several types of SQL collation, including:
Case sensitivity, which distinguishes between uppercase and lowercase characters
Accent sensitivity, which distinguishes between accented and unaccented characters
Width sensitivity, which distinguishes between full-width and half-width characters
The MOD function can be used in most RDBMSs as part of the WHERE statement in a select query to retrieve odd or even data entries in a table.
The formatting is as follows:
For even numbers, use ‘MOD (column name, 2) = 1’
For odd numbers, use ‘MOD (column name, 2) = 0’
A column can be renamed by following these steps:
Use ‘ALTER TABLE table name’ to select the table with the column you want to rename
Use ‘RENAME COLUMN old name to new name’ to rename the column
The main differences between the DELETE and TRUNCATE commands are:
DELETE is a DML command, whereas TRUNCATE is a DDL command
DELETE removes records and records each deletion in the transaction log, whereas TRUNCATE deallocates pages and records each deallocation in the transaction log
TRUNCATE is generally considered quicker as it makes less use of the transaction log
Data from one table can be copied into another by following these steps:
Use the INSERT INTO statement and specify the destination as the new table
Use the SELECT statement to specify the columns to copy (select all if you want to copy the complete table)
Use the WHERE statement to specify the table you want to copy
Finding data entries belonging to a particular group (in this case, employees belonging to a particular department) can be achieved in a few ways. These include:
Use SELECT, FROM, and WHERE statements
Use SELECT, FROM, GROUP BY, and HAVING statements
Use SELECT, FROM, INNER JOIN, and WHERE statements
A column in a table can be deleted by following these steps:
Use ‘ALTER TABLE table name’ to select the table with the column you want to delete
Use ‘DROP COLUMN column name’ to select the column you want to delete
You can retrieve data entries beginning with a particular letter using the LIKE command by following these steps:
Use the SELECT statement to specify the column with the names you want to vet
Use the FROM statement to specify the table containing that column
Use ‘WHERE column name’ followed by ‘LIKE x%’, with x representing the letter you are searching for
Use ‘ORDER by column name’ to complete the query
Aggregate values are used to perform calculations on a set of values to return a single value. Some of the most widely used aggregate functions are:
AVG calculates the average set of values
COUNT counts the total number of rows in a table
MIN finds the minimum value in a table
MAX finds the maximum value in a table
SUM calculates the sum of the values
A schema refers to a collection of database objects—such as tables, functions, indexes, and procedures—associated with a database.
The schema helps segregate database objects for different applications and access rights; it’s generally used to define who can and who cannot view specific objects in the database.
Scalar functions are user-defined functions applied to a set of data to return a single value. Some of the most common scalar functions include:
UCASE converts values to uppercase
LCASE converts values to lowercase
MID extracts textual data based on specified criteria
ROUND rounds numerical data to a specified number of decimals
NOW returns the current system date and time
We recommend using the advanced SQL interview questions below when recruiting for senior SQL roles, where candidates have at least a few years of experience with SQL.
An SQL injection is a type of cyber attack in which hackers insert malicious SQL code into the database to gain access to potentially valuable or sensitive information. It’s a fairly common occurrence with web applications or websites that use an SQL-based database.
It’s possible to prevent SQL injections by creating multiple database accounts to limit access or by using a third-party web application firewall.
There are several ways to optimize queries and improve performance. For example:
Specify particular columns with SELECT rather than by using SELECT * (select all)
Make joins with INNER JOIN rather than WHERE
Define filters using WHERE rather than HAVING
Avoid looping statements in the query structure
Avoid correlated subqueries
The process of SQL normalization can be divided into six steps or types. These include:
First Normal Form (1NF) to ensure rows and columns always contain singular, unique values
Second Normal Form (2NF) to remove all partial dependencies
Third Normal Form (3NF) to remove all transitive functional dependencies
Boyce-Codd Normal Form (BCNF) or Fourth Normal Form (4NF) to ensure all functional dependencies are a super key to the table
Fifth Normal Form (5NF) to ensure decomposition doesn’t result in any loss of data
Sixth Normal Form (6NF) to decompose the relation variables into irreducible components
ACID is an acronym for Atomicity, Consistency, Isolation, and Durability. These are the four key properties for ensuring data integrity during a transaction.
The role of each property is as follows:
Atomicity: Changes to data are performed as a single, unified operation
Consistency: Data values are consistent at the start and end of the transaction
Isolation: The intermediate state of a transaction is hidden from other transactions
Durability: Changes to data remain the same after the transaction is completed
Stored procedures are chunks of SQL code that can be saved and reused. The main types of stored procedures are:
User-defined stored procedures, which are created by users
System stored procedures are default procedures placed permanently on the system
Temporary stored procedures are procedures that are dropped when the session is closed
Remote stored procedures, which are created and stored on remote servers
To find employees with the same salary, the following solution can be used:
Use the SELECT statement to specify the relevant table
Use the FROM statement to specify the employee column
Use the WHERE statement to specify the salary criteria, for example:
‘WHERE salary IN
WHERE employee.employee_id <> employee.employee_id)’
There are several ways to remove duplicate rows from a table. These include:
Using Common Table Expressions (CTE) with the ROW_NUMBER function to identify and remove duplicate rows
Using the RANK function with the PARTITION BY clause
Using the GROUP BY clause with the COUNT function, and then replacing SELECT with DELETE FROM
A trigger is a type of stored procedure that runs when a specific event occurs, such as when a new record is added to the database.
Trigger creation varies depending on the RDBMS. Some systems feature a CREATE TRIGGER statement, while others require the users to navigate to a triggers folder in the toolbar. Once created, users must write the trigger’s code, specifying its conditions and effect.
The key differences between HAVING and WHERE SQL clauses are:
The WHERE clause is used in row operations, whereas the HAVING clause is used in column operations
The WHERE clause comes before GROUP BY in a query, whereas the HAVING clause comes after GROUP BY
The WHERE clause cannot be used with aggregate functions, contrary to the HAVING clause
Encrypting databases with sensitive information is crucial for security reasons. Encryption can be implemented in several ways. For example, you could:
Create a master key
Use symmetric encryption
Create a certificate protected by the master key
A correlated subquery can be used to find the sixth highest-earning employee in a table. The steps are as follows:
Use the SELECT statement to specify the employee name and salary columns
Use the FROM statement to specify the relevant table
Use the WHERE statement to specify the salary criteria, for example:
‘WHERE statement 6-1 = (SELECT COUNT(DISTINCT salary) FROM #Employee e2
WHERE e2.salary > e1.salary)’
There are several methods to avoid duplicate entries when making a query, such as to:
Create a unique index
Add the DISTINCT keyword to the SELECT statement
Use the NOT EXISTS or NOT IN commands
The key differences between an implicit and explicit cursor are:
Implicit cursors are created automatically when select statements are executed, whereas explicit cursors need to be defined explicitly by the user
Implicit cursors can only retrieve data from a single row at a time, whereas explicit cursors can retrieve data from multiple rows
Implicit cursors are less efficient and more error-prone compared to explicit cursors
Online Transaction Processing (OLTP) and Online Analytical Processing (OLAP) are two different data processing systems within the data science field.
OLTP systems capture, store, and update data regularly, whereas OLAP systems are used to query and perform analysis on that data.
Dynamic SQL is a programming technique used to build SQL statements at runtime, rather than at compile-time. Dynamic SQL is more challenging and less efficient than static SQL, but it allows developers to create more flexible, general-purpose applications.
Three types of relationships can exist between a pair of database tables. These are:
One-to-one, where each record in a table can be related to one record in the paired table
One-to-many, where each record in a table can be related to one or more records in the paired table
Many-to-many, where each record in both tables can be related to one or more records in the paired table
The interview is one of the last and most important stages in the recruitment process. In it, you assess both soft and hard skills, so you need to choose your interview questions wisely: they give you the opportunity to assess candidates’ SQL skills and behavior. Finding the right match on both fronts is imperative.
The questions above cover the first part, and allow you to accurately assess candidates’ SQL coding skills.
SQL requirements will vary depending on the database system used by your organization. With this in mind, tailor your questions where possible to the requirements of the role. It’s also possible to tailor your selection process in its early stages by incorporating a targeted SQL skills assessment.
SQL skills assessments are an invaluable resource as they allow you to gauge the candidate’s competency first-hand—something that interview questions are unable to offer. TestGorilla offers several SQL assessments tailored to different business requirements and ability levels:
A low-complexity SQLite Database Operations test for entry-level positions
A medium-complexity SQLite Querying test for mid-level positions
A high-complexity SQLite Querying test for senior-level positions
Skills assessments are best used after initial application screening has taken place, but they can even replace this step, as they allow you to quickly filter out unsuitable candidates.
You can follow them with an informal phone interview and a role-specific task for shortlisted applicants. This system helps you select the best candidates for an interview, and reduce time to hire and cost per hire.
By implementing a structured interview as part of a rigorous, multi-stage selection process, you can identify the right SQL specialist for your company.
Remember: the objective of recruitment isn’t to find the candidate with the broadest SQL skillset, but the one that best matches your business requirements.
Finding that perfect fit will transform your data analysis strategy, enabling your organization to stay ahead of the curve for years to come.
TestGorilla’s SQL tests can help you find the right SQL developers fast and bias-free. Get started for free today and start making better hiring decisions.
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