Mastering Google SQL Coding Interviews: A Comprehensive Collection of Interview Questions

As one of the tech giants in the world, Google is known for its rigorous interview process, particularly in roles that involve data analysis and manipulation. SQL, or Structured Query Language, is a fundamental tool in the realm of data management, and mastering it can significantly increase your chances of landing your dream job at Google. In this article, we’ll dive into a collection of Google SQL coding interview questions, providing you with the necessary preparation to tackle these challenges with confidence.

Understanding Google’s SQL Interview Process

Before we delve into the questions, let’s briefly discuss the SQL interview process at Google. SQL questions typically arise during the technical screen and on-site interview stages. During the technical screen, you can expect basic SQL questions, including definitions of concepts like JOIN, GROUP BY, and DATE functions. The on-site interview will likely involve more advanced SQL questions, requiring you to write complex queries using CASE statements, subqueries, and JOIN clauses.

A Comprehensive Collection of Google SQL Coding Interview Questions

1. Explain the different types of joins in SQL.

   • INNER JOIN: Returns rows from both tables where the join condition is met.
   • LEFT JOIN: Returns all rows from the left table and matching rows from the right table.
   • RIGHT JOIN: Returns all rows from the right table and matching rows from the left table.
   • FULL JOIN: Returns all rows from both tables, regardless of whether the join condition is met or not.

2. What is the PRIMARY KEY in SQL?
   A PRIMARY KEY is a column or a combination of columns that uniquely identifies each record in a table. It enforces entity integrity by ensuring that no duplicate values can be stored in the designated column(s). Primary keys cannot have NULL values, and each value must be unique.

3. What are constraints?
   Constraints in SQL are rules or conditions that are applied to data columns within a table. They help maintain data integrity and consistency. Some common constraints include:

   • NOT NULL: Ensures that a column cannot have NULL values.
   • UNIQUE: Ensures that all values in a column are unique.
   • PRIMARY KEY: A combination of NOT NULL and UNIQUE constraints, ensuring that each row in a table is uniquely identified.
   • FOREIGN KEY: Establishes a link between two tables, ensuring referential integrity.

4. What’s the difference between DELETE and TRUNCATE statements in SQL?

   • DELETE is a Data Manipulation Language (DML) statement that removes specific rows from a table based on a specified condition. It is a row-level operation, and the deleted data can be rolled back.
   • TRUNCATE is a Data Definition Language (DDL) statement that removes all rows from a table. It is a table-level operation, and the data cannot be rolled back. TRUNCATE is generally faster than DELETE as it does not log individual row deletions.

5. What is query optimization?
   Query optimization is the process of restructuring SQL queries to improve their performance and efficiency. It involves analyzing the query structure, indexing strategies, and data distribution to find the most optimal execution plan. Query optimization aims to minimize resource utilization (CPU, memory, I/O) and reduce query execution time, ultimately enhancing database performance.

6. Given the tables below, select the top three departments with at least ten employees and rank them according to the percentage of their employees making over $100,000 in salary.

   sql

   Copy

   employees tableColumns: id, first_name, last_name, salary, department_iddepartments tableColumns: id, name

   Expected output:

   percentage_over_100K	department name	number of employees
   0.9	engineering	25
   0.5	marketing	50
   0.12	sales	12

7. Given a users table, write a query to get the cumulative number of new users added by day, with the total reset every month.

   sql

   Copy

   users tableColumns: id, name, created_at

   Expected output:

   Date	Monthly Cumulative
   2020-01-01	5
   2020-01-02	12
   …	…
   2020-02-01	8
   2020-02-02	17
   2020-02-03	23

8. Given a table of product subscriptions with a subscription start date and end date for each user, write a query that returns true or false whether or not each user has a subscription date range that overlaps with any other user.

   sql

   Copy

   subscriptions tableColumns: user_id, start_date, end_date

   Example input:

   user_id	start_date	end_date
   1	2019-01-01	2019-01-31
   2	2019-01-15	2019-01-17
   3	2019-01-29	2019-02-04
   4	2019-02-05	2019-02-10

   Expected output:

   user_id	overlap
   1	1
   2	1
   3	1
   4	0

9. Given a table of students and their SAT test scores, write a query to return the two students with the closest test scores with the score difference. If there are multiple students with the same minimum score difference, select the student name combination that is higher in the alphabet.

   sql

   Copy

   scores tableColumns: id, student, score

   Input:

   id	student	score
   1	Jack	1700
   2	Alice	2010
   3	Miles	2200
   4	Scott	2100

   Output:

   one_student	other_student	score_diff
   Alice	Scott	90

10. We’re given two tables, a users table with demographic information and the neighborhood they live in and a neighborhoods table. Write a query that returns all of the neighborhoods that have 0 users.

    sql

    Copy

    users tableColumns: id, name, neighborhood_id, created_atneighborhoods tableColumns: id, name, city_id

    Expected output:

    neighborhood_name
    [Neighborhood names with 0 users]

11. Given a table of transactions and products, write a query to return the product id, product price, and average transaction price of all products with price greater than the average transaction price.

    sql

    Copy

    transactions tableColumns: id, user_id, created_at, product_id, quantityproducts tableColumns: id, name, price

12. Let’s say we have two tables, transactions and products. Hypothetically, the transactions table consists of over a billion rows of purchases bought by users. We are trying to find paired products that are often purchased together by the same user, such as wine and bottle openers, chips and beer, etc. Write a query to find the top five paired products and their names. Note: for the purposes of satisfying the test case, P1 should be the item that comes first in the alphabet.

    sql

    Copy

    transactions tableColumns: id, user_id, created_at, product_id, quantityproducts tableColumns: id, name, price

    Expected output:

    P1	P2	count
    [Product name 1]	[Product name 2]	[Count of paired purchases]
    …	…	…

By practicing and mastering these Google SQL coding interview questions, you’ll be well-prepared to showcase your SQL proficiency and stand out in the competitive interview process at Google. Remember, preparation is key, and dedicating time to hone your SQL skills will undoubtedly increase your chances of success.