Hiring guide for SQL Engineers

SQL Developer Hiring Guide

SQL, which stands for Structured Query Language, is a programming language used to manage and manipulate databases. It is particularly effective for handling structured data, i.e., data incorporating relations among entities and variables. SQL allows users to create, retrieve, update and delete database records. It's not only a language but also an interface between a user and the database system. SQL includes features like comprehensive support for distributed databases, high flexibility with indexing & views as well as robust transaction control mechanisms - all of which make it widely used in both small scale applications & large enterprise systems across industries.

Ask the right questions secure the right SQL talent among an increasingly shrinking pool of talent.

First 20 minutes

General SQL app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in SQL application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

How would you explain the difference between SQL and NoSQL databases?
SQL databases are relational, use structured query language and have a predefined schema. They are suitable for complex queries. NoSQL databases are non-relational, do not use structured query language and have dynamic schema. They are suitable for hierarchical data storage.
What are the different types of SQL commands?
The different types of SQL commands are DDL (Data Definition Language), DML (Data Manipulation Language), DCL (Data Control Language), and TCL (Transaction Control Language).
Describe the difference between a primary key and a unique key.
A primary key uniquely identifies each record in a table and must not contain NULL values. A unique key also uniquely identifies a record in a table, but it can contain NULL values.
How would you retrieve all the records from a table?
To retrieve all records from a table, you would use the SELECT command followed by an asterisk. For example, 'SELECT * FROM table_name'.
What is the purpose of the GROUP BY statement in SQL?
The GROUP BY statement is used with aggregate functions to group the result-set by one or more columns.
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What you’re looking for early on

Does the candidate demonstrate a strong understanding of SQL syntax and commands?
Has the candidate shown experience with database design and normalization?
Is the candidate familiar with SQL Server or other relevant database management systems?
Can the candidate explain complex SQL concepts in simple terms?

Next 20 minutes

Specific SQL development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

Describe the difference between the DELETE and TRUNCATE commands.
The DELETE command is used to remove rows from a table based on the condition that we provide with a WHERE clause. TRUNCATE is a DDL (Data Definition Language) operation that is used to mark the extents of a table for deallocation (empty for reuse). The result of this operation quickly removes all data from a table, typically bypassing a number of integrity enforcing mechanisms intended to protect the data.
How would you select unique records from a table?
The SELECT DISTINCT statement is used to return only distinct (different) values. For example, 'SELECT DISTINCT column_name FROM table_name'.
What does the SQL INNER JOIN keyword do?
The INNER JOIN keyword selects records that have matching values in both tables.
Describe the difference between SQL and PL/SQL.
SQL is a data oriented language for selecting and manipulating sets of data. PL/SQL is a procedural language to create applications. PL/SQL can include SQL commands.
How would you find the second highest salary from a table?
You can use the following SQL subquery to find the second highest salary: 'SELECT MAX(Salary) FROM Employee WHERE Salary NOT IN (SELECT MAX(Salary) FROM Employee)'.
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The ideal back-end app developer

What you’re looking to see on the SQL engineer at this point.

At this point, a skilled SQL engineer should demonstrate strong problem-solving abilities, proficiency in SQL programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with SQL.

What does the following SQL statement do?
SELECT COUNT(*) FROM Employees WHERE Salary > 50000;
The SQL statement counts the number of employees who have a salary greater than 50000.
What is the output of the following SQL statement?
SELECT MAX(Salary) FROM Employees;
The SQL statement returns the maximum salary from the Employees table.
What does the following SQL statement do?
SELECT DISTINCT Department FROM Employees;
The SQL statement selects all unique departments from the Employees table.
What will be the output of the following SQL statement?
SELECT AVG(Salary) FROM Employees WHERE Department = 'HR';
The SQL statement returns the average salary of employees in the HR department.

Wrap-up questions

Final candidate for SQL Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle SQL application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

What are the different types of subquery?
The different types of subquery include single-row subquery (returns only one row), multiple-row subquery (returns multiple rows), and multiple column subquery (returns multiple columns).
Describe the difference between a local and a global temporary table.
A local temporary table exists only for the duration of a connection or, if defined inside a compound statement, for the duration of the compound statement. A global temporary table remains in the database permanently, but the rows exist only within a given connection.
How would you update a record in a database?
To update a record in a database, you would use the UPDATE statement. For example, 'UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition'.

SQL application related

Product Perfect's SQL development capabilities

Beyond hiring for your SQL engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in SQL projects, and can engage in multiple capacities.