Essential SQL Interview Questions for Data Engineers Quiz

Test your knowledge of SQL fundamentals, functions, joins, optimization, and advanced concepts with this beginner-friendly quiz designed for aspiring data engineers. Strengthen your SQL skills with real-world questions suitable for interview preparation.

1. Understanding SQL Basics

   Which of the following best describes a primary key in SQL?

   1. A command to delete tables from a database
   2. A unique identifier for each row in a table
   3. A statement to group data by a specific column
   4. A rule that prevents updates in a table

   Explanation: A primary key uniquely identifies each row within a table and ensures no duplicate values are present in that column. Deleting tables or updating rules are unrelated to primary keys. Grouping data is done using GROUP BY and not by primary keys.

2. SQL Statement Usage

   If you want to retrieve all rows and columns from a table named 'employees', which SQL statement should you use?

   1. SELECT * FROM employees;
   2. GET EVERY employees FROM *;
   3. MAKE SELECT employees;
   4. CHOOSE ALL FROM employees;

   Explanation: The correct SQL query is 'SELECT * FROM employees;'. The syntax uses SELECT followed by an asterisk to choose all columns, and FROM to specify the table name. The other options are incorrect SQL syntax or commands that do not exist in SQL.

3. SQL Clauses

   What is the main difference between the WHERE and HAVING clauses in SQL?

   1. WHERE adds columns, HAVING deletes rows
   2. WHERE filters rows before grouping, HAVING filters after grouping
   3. WHERE groups rows, HAVING aggregates columns
   4. WHERE sorts data, HAVING merges tables

   Explanation: WHERE is used to filter rows before grouping occurs, while HAVING is used to filter data after GROUP BY has been applied. The other options describe unrelated tasks such as sorting, merging, or incorrectly defining the clauses.

4. SQL Functions

   Which SQL function would you use to combine two strings into one, such as joining first and last names?

   1. DIVIDE()
   2. CONCAT()
   3. FORMAT()
   4. MATCH()

   Explanation: The CONCAT() function is used to join two or more strings together in SQL, making it ideal for creating full names from separate columns. DIVIDE() and FORMAT() are unrelated to string joining, while MATCH() is not used for concatenating values.

5. NULL Value Handling

   If a column has NULL values, which function can you use in SQL to replace them with a default value?

   1. UCASE()
   2. COALESCE()
   3. INCLUDE()
   4. DISTINCT()

   Explanation: COALESCE() is designed to return the first non-NULL value in a list, making it ideal to replace NULLs with defaults. UCASE() is for converting text to uppercase, DISTINCT() selects unique values, and INCLUDE() is not a SQL function.

6. Join Types

   Which type of join returns only the rows where there is a match in both joined tables?

   1. INNER JOIN
   2. FULL OUTER JOIN
   3. CROSS JOIN
   4. RIGHT JOIN

   Explanation: INNER JOIN retrieves rows where there is a matching value in both tables, ensuring only common records appear. FULL OUTER JOIN includes all records with or without matches, RIGHT JOIN shows all from the right table, and CROSS JOIN forms combinations regardless of matches.

7. SQL Aggregate Functions

   Which aggregate function in SQL returns the highest value from a specified column?

   1. SUM()
   2. MAX()
   3. MIN()
   4. AVG()

   Explanation: MAX() returns the largest value from the column specified. MIN() returns the lowest, SUM() adds up all numbers, and AVG() returns the average. Only MAX() provides the highest numerical or alphabetical value.

8. Pattern Matching

   To find all usernames starting with 'A', which SQL clause would you most commonly use?

   1. LIKE 'A%'
   2. COUNT('A%')
   3. GROUP BY 'A%'
   4. SORT BY 'A%'

   Explanation: The LIKE clause is used for pattern matching, and 'A%' means any string starting with 'A'. GROUP BY is used for aggregation, COUNT() returns counts, and SORT BY is used for ordering, not matching patterns.

9. Date Functions

   Which function helps extract the year value from a date column in SQL?

   1. YEAR()
   2. ROUND()
   3. INDEX()
   4. RANK()

   Explanation: YEAR() extracts the year part from a date. ROUND() is for numerical rounding, INDEX() deals with database structure, and RANK() is used for ranking rows. None except YEAR() is related to dates.

10. Operators Usage

    If you want to check if a value exists within a set of multiple possible values, which operator should you use in SQL?

    1. UPPER
    2. IN
    3. BETWEEN
    4. JOIN

    Explanation: 'IN' checks if a value matches any value within a list. 'BETWEEN' checks ranges, 'JOIN' is for combining tables, and 'UPPER' is a string function for capitalization, not comparisons.

11. Index Concepts

    How do SQL indexes generally improve database performance?

    1. By allowing faster searches for specific rows
    2. By increasing the number of allowed columns
    3. By encrypting sensitive data
    4. By automatically creating backups

    Explanation: Indexes create a quick lookup reference, enabling databases to search for rows much faster than scanning all data. They do not increase the column count, create backups, or encrypt data; those are different database features.

12. SQL Views

    What is a view in SQL?

    1. A virtual table based on a SQL query
    2. A type of primary key
    3. A backup of a physical table
    4. A row with special permissions

    Explanation: A view is a stored query that behaves like a table for reading data but doesn’t store data itself. A backup is a copy of a table's actual data, not a view. Rows with permissions and primary keys are unrelated concepts.

13. Grouping Data

    Which clause is used in SQL to arrange identical data into groups?

    1. FORMAT BY
    2. GROUP BY
    3. WHERE
    4. ORDER BY

    Explanation: GROUP BY groups rows sharing the same values in specified columns, often used with aggregate functions. ORDER BY sorts data, WHERE filters data, and FORMAT BY is not a SQL clause.

14. Transactions

    What is the main purpose of a transaction in SQL databases?

    1. To backup the entire database
    2. To automatically add foreign keys to tables
    3. To sort data alphabetically in a table
    4. To ensure a series of SQL statements execute as a single, complete unit

    Explanation: Transactions guarantee all included operations succeed or fail together, ensuring data consistency. Sorting data, adding keys, and backups are unrelated tasks not covered by transactions.

15. Data Normalization

    What is the primary goal of normalization in SQL databases?

    1. To increase the number of tables without organizing them
    2. To create graphical reports automatically
    3. To speed up transaction execution
    4. To reduce data redundancy and improve data integrity

    Explanation: Normalization is designed to minimize repetition and ensure data is organized efficiently, supporting integrity. Increasing tables without structure, automatic reports, and transaction speed are not direct goals of normalization.