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To land your dream job as a data engineer, you need to master the Structured Query Language (SQL) interview questions. SQL is incredibly useful when you’re dealing with large amounts of data, want to retrieve specific results, or draw insights quickly. No wonder that it is extremely popular among data scientists/engineers and anyone involved in data work.
Being skilled at SQL and excellent at answering SQL query interview questions is especially crucial if your dream job is to be a data engineer. This article will help you assess your SQL interview prep and clear some of the concepts.
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In this article, we’ll learn:
SQL Query: Introduction and Usage
SQL is the most common language used to extract and organize data stored in a relational database. SQL allows you to:
- Execute queries
- Retrieve data
- Insert, update, and delete data
- Create new databases, tables, views, and stored procedures
- Set access permissions and more!
Let’s jump right in and look at some sample SQL/SQL query-related interview questions and answers to see its use in action.
SQL Query Interview Questions and Answers for Practice
- Which TCP/IP port does the SQL Server run?
While TCP/IP of an SQL server can be changed, by default, it runs on port number 1433. - What is the order of clauses in an SQL SELECT statement?
The order goes: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY. Here, everything other than SELECT and FROM is optional. Only SELECT and FROM are mandatory. - How would you select random rows from a table?
We can select random rows in SQL using a SAMPLE clause as follows:
SELECT * FROM table_name SAMPLE(5);
- How will you rename a column in the output of the SQL query?
To rename a column in the output of an SQL query, we can make use of the AS keyword as follows:
SELECT old_column_name AS new_column_came FROM table_name;
SQL Query Interview Questions Based on a Sample Data Table
Here’s a sample table: Employee
We’ll now look at some sample SQL query questions and answers:
- Write an SQL query that gets the first two characters of last_name from the Employee table.
Select substring(last_name,1,2) from Employee;
- Write an SQL query that gets the department from the Employee table after replacing ‘e’ with ‘Z.’
Select REPLACE(department,'e','Z') from Employee;
- Write an SQL query that gets the first and last names from the Employee table together separated by a space in a single column full_name.
Select CONCAT(first_name, ' ', last_name) AS 'full_name' from Employee;
There can be numerous questions that can be made by changing constraints and requirements. Some common constraints and requirements include:
- Printing a column after removing white spaces from the left or right side.
- Getting unique values only from a column and avoiding duplicates
- Printing in ascending or descending order
- Print all details associated with a specific employeeID, name, etc.
- Print all details associated with an employeeID, name, etc. that starts or ends with a given character
- Print the names of employees with IDs/salaries within a specified range.
More SQL Query Interview Questions
Here are some more SQL query interview questions to help you evaluate your preparation:
Top Basic Interview Questions on SQL Queries for Testers
1. In the context of SQL, define:
- Join, right join, outer join, full join, cross join, self join
- Scalar functions
- Indexes
- Transactions
- Identity
- View and Table
- Privileges
- SQL injection
- NVL function
- Cartesian product of a table
- Normalization
- DDL, DML, and DCL
- Clustered indexes and Non-clustered indexes
- A relationship
- Stored Procedures
- Triggers and Nested Triggers
- Cursor
- Collation
- Database testing, Database Whitebox Testing, and Database Blackbox Testing
- SELECT, FROM, WHERE
- CREATE, ALTER
- DELETE, TRUNCATE, DROP
- COMMIT, ROLLBACK, SET TRANSACTION, SAVEPOINT
- Primary key and foreign key
- CHECK constraint
2. What values can BOOLEAN data field take?
3. Can a table have more than one foreign key? What about a primary key? Why?
SQL Query Interview Questions for Experienced Professionals: Complex SQL Queries
1. Name and explain all the different types of:
- Joins in SQL
- Transaction controls
- Aggregate functions
- Privileges
- Constraints
- DDL statements
- DML statements
- DCL statements
- SQL clauses
- Scalar functions
- Relationships
- Normalization forms
2. Explain and differentiate between:
- SQL, PL/SQL, Oracle SQL, and MySQL
- Table and view
- DELETE, DROP, and TRUNCATE
- Clustered and non-clustered indexes
- Database white box and black box testing
- Right join, outer join, full join, cross join, and self join
3. How can we update a view in SQL?
4. How do SQL Privileges work?
5. Describe some properties of relational databases.
To understand the type of SQL interview questions asked at top tech companies, read:
SQL Queries Topics to Prepare and Tips
Whether you’re working with PL/SQL, MySQL, or OracleSQL, you should thoroughly prepare and practice interview questions related to these SQL query topics for your next interview:
- SQL query interview questions related to joins
- SQL query interview questions related to commands, normalization, etc.
- Differentiate between and explain concept questions
- Query types, aliases, functions, conditions, comparison operators, and other such concepts.
You should also take a sample table and solve all possible SQL query interview questions you can make out of it.
For more questions, read this article on SQL interview questions and tips on cracking them.
SQL Interview Question FAQs
1. What are the different types of statements supported by SQL?
There are three types of statements supported by SQL — Data Definition Language (DDL), Data Manipulation Language (DML), and Data Control Language (DCL).
2. What is the difference between DDL, DML, and DCL in SQL?
Data Definition Language statements are used to define the database structures like tables. Example of DDL statements: CREATE, ALTER, and DROP. Data Manipulation Language statements are used to manipulate the data stored in the records. Example of DML statements: INSERT, UPDATE, and DELETE. (SELECT is a partial DML statement). Data Control Language statements are used to set privileges that give, revoke, or limit access to databases for specific users. Example of DCL statements: GRANT, REVOKE.
3. How do we use the DISTINCT statement in SQL? What is its use?
The DISTINCT statement is used along with the SELECT statement. If there are duplicate values in the record, the DISTINCT statement is used to select only all unique values from the record to avoid duplicates.
4. What are the different Clauses used in SQL?
The different clauses used in SQL are: WHERE, GROUP BY, HAVING, ORDER BY, and USING.
5. Why do we use SQL constraints? Which constraints can we use while creating a database in SQL?
Constraints set the rules for all the records in the table, so any action that violates the constraints is aborted. Constraints can be declared at the time of table creation using CREATE TABLE or later using ALTER TABLE.
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