SQL 50: Data Analyst Interview Prep

This is a walkthrough of the 50 SQL problems that show up most often when people prep for data analyst, BI, and data analytics intern interviews. It's based on LeetCode's free "SQL 50" study plan, and every solution here is written in MySQL.

Structure

sql-50-data-analyst-interview-prep/
├── README.md
├── LICENSE
├── STUDY_GUIDE.md
├── PROGRESS.md
└── solutions/
    ├── 01-filtering/
    │   ├── 1757-recyclable-and-low-fat-products.sql
    │   ├── 584-find-customer-referee.sql
    │   ├── 595-big-countries.sql
    │   ├── 1148-article-views-i.sql
    │   ├── 1683-invalid-tweets.sql
    │   ├── 620-not-boring-movies.sql
    │   ├── 610-triangle-judgement.sql
    │   ├── 1527-patients-with-a-condition.sql
    │   ├── 1667-fix-names-in-a-table.sql
    │   └── 1517-find-users-with-valid-e-mails.sql
    ├── 02-joins/
    │   ├── 1378-replace-employee-id-with-the-unique-identifier.sql
    │   ├── 1068-product-sales-analysis-i.sql
    │   ├── 1581-customer-who-visited-but-did-not-make-any-transactions.sql
    │   ├── 577-employee-bonus.sql
    │   ├── 1280-students-and-examinations.sql
    │   ├── 1075-project-employees-i.sql
    │   ├── 1731-the-number-of-employees-which-report-to-each-employee.sql
    │   └── 196-delete-duplicate-emails.sql
    ├── 03-aggregation/
    │   ├── 570-managers-with-at-least-5-direct-reports.sql
    │   ├── 596-classes-more-than-5-students.sql
    │   ├── 1729-find-followers-count.sql
    │   ├── 619-biggest-single-number.sql
    │   ├── 1045-customers-who-bought-all-products.sql
    │   ├── 2356-number-of-unique-subjects-taught-by-each-teacher.sql
    │   ├── 1484-group-sold-products-by-the-date.sql
    │   ├── 1327-list-the-products-ordered-in-a-period.sql
    │   ├── 1907-count-salary-categories.sql
    │   └── 585-investments-in-2016.sql
    ├── 04-subqueries/
    │   ├── 176-second-highest-salary.sql
    │   ├── 185-department-top-three-salaries.sql
    │   ├── 197-rising-temperature.sql
    │   ├── 180-consecutive-numbers.sql
    │   ├── 1070-product-sales-analysis-iii.sql
    │   ├── 1164-product-price-at-a-given-date.sql
    │   ├── 1789-primary-department-for-each-employee.sql
    │   └── 1978-employees-whose-manager-left-the-company.sql
    ├── 05-string-date-functions/
    │   ├── 1661-average-time-of-process-per-machine.sql
    │   ├── 1193-monthly-transactions-i.sql
    │   ├── 1141-user-activity-for-the-past-30-days-i.sql
    │   ├── 1211-queries-quality-and-percentage.sql
    │   ├── 1633-percentage-of-users-attended-a-contest.sql
    │   └── 1251-average-selling-price.sql
    └── 06-window-functions/
        ├── 1934-confirmation-rate.sql
        ├── 1174-immediate-food-delivery-ii.sql
        ├── 550-game-play-analysis-iv.sql
        ├── 1204-last-person-to-fit-in-the-bus.sql
        ├── 626-exchange-seats.sql
        ├── 1341-movie-rating.sql
        ├── 1321-restaurant-growth.sql
        └── 602-friend-requests-ii-who-has-the-most-friends.sql

Problems by category

Basic Filtering & SELECT

#	Problem	Difficulty	LeetCode	File
1	Recyclable and Low Fat Products	Easy	1757	1757-recyclable-and-low-fat-products.sql
2	Find Customer Referee	Easy	584	584-find-customer-referee.sql
3	Big Countries	Easy	595	595-big-countries.sql
4	Article Views I	Easy	1148	1148-article-views-i.sql
5	Invalid Tweets	Easy	1683	1683-invalid-tweets.sql
6	Not Boring Movies	Easy	620	620-not-boring-movies.sql
7	Triangle Judgement	Easy	610	610-triangle-judgement.sql
8	Patients With a Condition	Easy	1527	1527-patients-with-a-condition.sql
9	Fix Names in a Table	Easy	1667	1667-fix-names-in-a-table.sql
10	Find Users With Valid E-Mails	Easy	1517	1517-find-users-with-valid-e-mails.sql

Joins

#	Problem	Difficulty	LeetCode	File
11	Replace Employee ID With The Unique Identifier	Easy	1378	1378-replace-employee-id-with-the-unique-identifier.sql
12	Product Sales Analysis I	Easy	1068	1068-product-sales-analysis-i.sql
13	Customer Who Visited but Did Not Make Any Transactions	Easy	1581	1581-customer-who-visited-but-did-not-make-any-transactions.sql
14	Employee Bonus	Easy	577	577-employee-bonus.sql
15	Students and Examinations	Easy	1280	1280-students-and-examinations.sql
16	Project Employees I	Easy	1075	1075-project-employees-i.sql
17	The Number of Employees Which Report to Each Employee	Easy	1731	1731-the-number-of-employees-which-report-to-each-employee.sql
18	Delete Duplicate Emails	Easy	196	196-delete-duplicate-emails.sql

Aggregation, GROUP BY & HAVING

#	Problem	Difficulty	LeetCode	File
19	Managers with at Least 5 Direct Reports	Medium	570	570-managers-with-at-least-5-direct-reports.sql
20	Classes More Than 5 Students	Easy	596	596-classes-more-than-5-students.sql
21	Find Followers Count	Easy	1729	1729-find-followers-count.sql
22	Biggest Single Number	Easy	619	619-biggest-single-number.sql
23	Customers Who Bought All Products	Medium	1045	1045-customers-who-bought-all-products.sql
24	Number of Unique Subjects Taught by Each Teacher	Easy	2356	2356-number-of-unique-subjects-taught-by-each-teacher.sql
25	Group Sold Products By The Date	Easy	1484	1484-group-sold-products-by-the-date.sql
26	List the Products Ordered in a Period	Easy	1327	1327-list-the-products-ordered-in-a-period.sql
27	Count Salary Categories	Medium	1907	1907-count-salary-categories.sql
28	Investments in 2016	Medium	585	585-investments-in-2016.sql

Subqueries & Advanced SELECT

#	Problem	Difficulty	LeetCode	File
29	Second Highest Salary	Medium	176	176-second-highest-salary.sql
30	Department Top Three Salaries	Hard	185	185-department-top-three-salaries.sql
31	Rising Temperature	Easy	197	197-rising-temperature.sql
32	Consecutive Numbers	Medium	180	180-consecutive-numbers.sql
33	Product Sales Analysis III	Medium	1070	1070-product-sales-analysis-iii.sql
34	Product Price at a Given Date	Medium	1164	1164-product-price-at-a-given-date.sql
35	Primary Department for Each Employee	Easy	1789	1789-primary-department-for-each-employee.sql
36	Employees Whose Manager Left the Company	Easy	1978	1978-employees-whose-manager-left-the-company.sql

String, Date & Formatting Functions

#	Problem	Difficulty	LeetCode	File
37	Average Time of Process per Machine	Easy	1661	1661-average-time-of-process-per-machine.sql
38	Monthly Transactions I	Medium	1193	1193-monthly-transactions-i.sql
39	User Activity for the Past 30 Days I	Easy	1141	1141-user-activity-for-the-past-30-days-i.sql
40	Queries Quality and Percentage	Easy	1211	1211-queries-quality-and-percentage.sql
41	Percentage of Users Attended a Contest	Easy	1633	1633-percentage-of-users-attended-a-contest.sql
42	Average Selling Price	Easy	1251	1251-average-selling-price.sql

Window Functions & Advanced Analytics

#	Problem	Difficulty	LeetCode	File
43	Confirmation Rate	Medium	1934	1934-confirmation-rate.sql
44	Immediate Food Delivery II	Medium	1174	1174-immediate-food-delivery-ii.sql
45	Game Play Analysis IV	Medium	550	550-game-play-analysis-iv.sql
46	Last Person to Fit in the Bus	Medium	1204	1204-last-person-to-fit-in-the-bus.sql
47	Exchange Seats	Medium	626	626-exchange-seats.sql
48	Movie Rating	Medium	1341	1341-movie-rating.sql
49	Restaurant Growth	Medium	1321	1321-restaurant-growth.sql
50	Friend Requests II: Who Has the Most Friends	Medium	602	602-friend-requests-ii-who-has-the-most-friends.sql

How to use this repo

• Each .sql file stands on its own: the schema, the task, a working solution, and a short explanation, all written as comments around one runnable query.
• Try solving the problem yourself first, ideally on LeetCode's Database problem set, then compare your answer against the file here.
• STUDY_GUIDE.md covers interview tips and a suggested order to work through the topics.
• PROGRESS.md is just a plain checklist. Fork it and check things off as you go.

License

MIT. See LICENSE for details.