14 Common SQL Interview Questions for Testers (With Example Answers)

October 13, 2020

Interviewing for a job as an SQL tester requires a high level of competency in the vast field of data science. While some interview questions may be more common, questions specific to database testing are far more in-depth. For that reason, practice and preparation are necessary to make a good impression on the interviewer. This article aims to provide examples of SQL tester questions and sample answers to help you prepare for your next interview. 

Example SQL tester interview questions 

Here are some common SQL tester interview questions: 

1. What is SQL?

“Structured Query Language is an American National Standards Institute database language used as a communication tool to create and access databases. SQL supports software applications by performing tasks such as retrieving, updating, inserting and deleting data. SQL makes it possible to execute queries, insert and update records, create and delete databases and tables, and more.”

2. What is a DBMS?

“DBMS stands for database management system, which is a program used to help maintain, monitor, create, deploy and control use of a database. It also acts as a file manager for the data kept in a database, no matter the size or complexity of the database.”

3. What are tables in SQL?

“The table is a collection of records organized into a set of rows and columns to form a model. There are no precise upper limits to the number of records that can be defined by rows in the table.”

4. What are the different clauses used in SQL?

“An SQL clause is defined for the purpose of limiting the queried results to certain specified conditions.

GROUP BY: used in aggregation to arrange identical data into groups, the GROUP BY clause follows the WHERE clause in a SELECT statement and is followed by the ORDER BY clause

HAVING: used to specify a search condition in a GROUP BY clause, HAVING can be used in the absence of a GROUP BY clause by using a WHERE clause

ORDER BY: sorts the result set in ascending (default) or descending (using DESC keyword) order

WHERE: used to define the condition of the records to be extracted”

5. What are the factors to check during database testing?

“Every functionality should be tested, including field size validation, constraints, indexing, stored procedures, data validity, data integrity, triggers and functions, and database performance, but checking the accuracy of added and deleted records should be monitored more closely.”

6. What is data-driven testing?

“Data-driven testing is an automation framework within which test data is stored in a table or spreadsheet, where input values are read from data files and stored in various test scripts. This is useful to avoid the need to create individual tests for each set of data, which can be a cumbersome process. The framework keeps the data quarantined, and the same test script can be used to generate results for multiple combinations of input test data. Input data can be stored as XLS, XML, CSV and database formats. There are four types of data-driven testing:

  • *Key-driven: Dynamic test data is submitted via keyboard to retest an application with new input values to validate the original calculation.*

  • *Flat files (.txt, .doc): Retesting using the data in a flat-file format.*

  • *Front-end object: Automation scripts are created by considering values like list, menu, table, data window and ocx.*

  • *Excel: This test runs the script for multiple inputs, which are stored in an Excel sheet.”*

7. What are JOINs in SQL, and what types of JOINs are there?

“A JOIN is a keyword used to retrieve data from multiple tables based upon the relationship between the fields of the tables and present the results as a single set. The various types of JOINs commonly include INNER JOIN, OUTER JOIN, LEFT JOIN and RIGHT JOIN.”

8. What is DML?

“DML stands for data manipulation language. DML is a computer programming language that facilitates the selection, insertion, deletion and updating of data in a database. DML parallels SQL in that it allows users to retrieve and use data in a relational database.”

9. What are DCL commands?

“DCL stands for Data Control Language and is a component of SQL that controls access to data stored in a database. The two types of DCL commands are:

Grant: grants users access to the database

Revoke: stops permitting users to access the database”

10. Does QTP support SQL queries?

“QTP doesn’t have built-in functionality for database connectivity, but VBScript language can be used to connect and interact with databases using ADODB objects, divided into four methods:

  • Connection: forms a connection with a database
  • Command: executes an SQL command
  • Fields: retrieves the correct column from a record
  • Recordset: retrieves data from a database”

11. How is data-load testing performed?

“You’ll need to have access to the source and destination databases.

  1. First, check the compatibility between the source database and destination database via the DTS package.

  2. Then, open the corresponding DTS package in SQL Enterprise Manager and run the DTS package.

  3. Compare the Source and the Target column data.

  4. Check the number to rows of Source and Target.

  5. Update the Source database, then check to make sure the changes have been reflected on the destination database.

  6. Check for NULLs.

  7. Finally, check the volume and response time of the database server in processing queries.”

12. What is retesting, and how is it different from data-driven testing?

“Retesting involves executing the same test with different input values after previously testing, finding the problem and repairing it. Retesting differs from data-driven testing in that retesting is a manual process where testing is done with a whole new data set, while data-driven testing is an automated process that tests the application with multiple sets of test data.”

13. What is performance testing?

“Performance testing is a highly specialized software testing technique that helps to determine how a system performs in terms of speed, sensitivity and stability under a heavy workload.”

14. What are SQL constraints? 

“Constraints are statements used to establish the rules for all records in the table. If any action violates a constraint, that action will be aborted. Constraints are defined while creating the database itself with CREATE TABLE statement, or after the table is created, by using ALTER TABLE statement. The five major constraints used in SQL are:

  • NOT NULL: indicates that the column is required to have some value; it cannot be left null

  • UNIQUE: ensures that each row and column has unique value; no value is being repeated in any other row or column

  • PRIMARY KEY: used in association with NOT NULL and UNIQUE constraints to identify a particular unique record

  • FOREIGN KEY: used to ensure the referential integrity of data in the table and also matches the value in one table with another using PRIMARY KEY

  • CHECK: used to ensure the value in columns obeys specified conditions”

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Tips for preparing for an SQL interview

Follow this guideline to prepare for your next SQL interview:

  • Research the company. Find out as much as you can about the company’s leadership, values and goals. Look through the company’s website, peruse its social media accounts and read any available press releases.

  • Make sure your skills match those required for the job. If there is a gap between them, consider refreshing your SQL knowledge, and craft an impressive answer to assure the interviewer that you are dedicated to continuously improving and expanding your skills.

  • Research SQL interview questions. Study recommended answers to common questions and craft your own succinct but deeply knowledgeable answers. 

  • Seek advice. Take notes, ask mentors and colleagues for tips and advice, and practice your most confident and relatable voice and posture.

  • Practice. Rehearse interview questions and your intended answers aloud, or recruit a friend to provide feedback. Practicing the interview can help you relax when the real interview happens, and it can also significantly improve the delivery of your responses.

Rehearsing for the interview should refresh your skills and knowledge, polish your response delivery and help you conquer the interview to get the job. Practice answering the technical questions above as well as general interview questions about you as a person. Remember to let your passion for your work show without being overly dramatic. Most importantly, be yourself. Practicing for the interview will help you present the best version of yourself.