MASTER SQL IN JUST 15 DAYS

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MASTER SQL IN JUST 15 DAYS

I had worked really hard to master SQL solving 300+ questions for over a month. This definitely helped me become really good at SQL. However, when I started giving interviews which included final rounds at Google, Apple and Amazon, I noticed 95% of questions always came from these 10 types of questions. Secondly, even for the theory questions , I noticed majority of the times that there were these 13 theory questions and that majority of SQL theoretical questions also came from these questions. So, I did 3 things :

1. Created a list of these 10 SQL Patterns and the problems to solve for these patterns.

2. Created answers for 13 SQL Theory Questions with Examples to ensure you can ace these questions too.

3. 15 Day Roadmap to master these SQL Interviews

Pattern 1 : Find duplicate records

• LeetCode — Duplicate Emails. Classic GROUP BY/HAVING duplicate detector. LeetCode

• DataLemur — Duplicate Job Listings (LinkedIn). Count companies posting duplicated jobs (same title/description). DataLemur

Pattern 2 : Top-N per group (window functions)

• LeetCode — Department Top Three Salaries (top-3 per department via DENSE_RANK). LeetCode

• DataLemur — Top Fans Rank (Spotify) (rank artists by top-10 appearances). DataLemur

• DataLemur — Highest Grossing Products (Amazon) (tutorial variant showing RANK for top spend per category). (closest match) staging.datalemur.com

Pattern 3 : Detect missing data / gaps (in IDs/dates)

(SQL platforms rarely ask pure “find gaps” with generated calendars; these are close practical matches that require detecting absence or coverage.)

• LeetCode — Sales Analysis III (identify products sold only within a fixed window—tests range coverage logic). (closest match) LeetCode

• DataLemur — IBM Db2 Product Analytics (build a histogram including employees with no queries during a period). (closest match) DataLemur

Pattern 4 : Second highest value

• LeetCode — Second Highest Salary. The archetype. LeetCode

• LeetCode — Nth Highest Salary (generalizes the same ranking trick). LeetCode

Pattern 5 : Pivot-style aggregations (wide columns)

(Most interview platforms use CASE…SUM to “pivot”.)

• DataLemur — Laptop vs. Mobile Viewership (NYTimes) (two columns: laptop_views & mobile_views). GitHub

• DataLemur — Time Spent on Snaps (Snapchat) (percent split columns via CASE/SUM). DataLemur

Pattern 6 : Running totals / cumulative metrics

• LeetCode — Game Play Analysis III (cumulative games_played per user by date). LeetCode

• DataLemur — Long Calls Growth (UnitedHealth) (uses LAG & monthly counts; pairs well with running totals thinking). DataLemur

Pattern 7 : Employee hierarchy / recursive CTEs

• DataLemur — Senior Managers (Google) (derive managers with their direct report counts; great CTE practice). DataLemur

Pattern 8 : Complex joins (inner/left/anti, multi-table)

• LeetCode — Trips and Users (join + filter out banned riders/drivers; daily cancel rate). LeetCode

• LeetCode — Customers Who Never Order (anti-join pattern). LeetCode

• LeetCode — Average Selling Price (date-range join across price intervals). LeetCode

Pattern 9 : Percentage contribution / share of total

• DataLemur — Time Spent on Snaps (Snapchat) (percent sending vs opening by age group). DataLemur

• DataLemur — Laptop vs. Mobile Viewership (NYTimes) (separate category totals—often extended to % of total). GitHub

Pattern 10 : Overlapping ranges (interval joins)

• LeetCode — Average Selling Price (join UnitsSold.purchase_date to [start_date, end_date] price windows). LeetCodeLeetcode

• DataLemur — Total Utilization Time (AWS) (aggregate overlapping server up-time intervals—classic interval logic). DataLemur

300+ SQL Interview Questions I SOLVED:

Here is the complete excel sheet of over 300+ questions for SQL which will help you ace any interview : Link.

Even practice 3 questions a day will help you master your skills.

20 MOST ASKED SQL THEORY QUESTIONS : Link

15 DAY BY DAY ROADMAP

Day 1–3: Core SQL Foundations (Theory from PDF)

(No coding yet — just theory practice)

Day 1

• SQL Command Types: DDL, DML, DCL, DQL.

• SQL Data Types and their usage.

• SELECT statement basics.

• Joins overview (Inner, Outer, Self, Cross).

Day 2

• Inner vs Outer Join (differences).

• Primary Key vs Foreign Key.

• Subquery vs Join.

• ACID Properties.

Day 3

• GROUP BY vs HAVING.

• SQL Order of Execution.

• UNION vs UNION ALL.

• Query Optimization techniques.

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Day 4–6: Advanced SQL Theory

Day 4

• CASE statement usage.

• Self Join (employee–manager).

• LIMIT clause.

Day 5

• Window Functions overview.

• RANK vs DENSE_RANK vs ROW_NUMBER.

• COUNT alternatives.

Day 6

• Second/Nth Highest Value.

• BETWEEN operator.

• Normalization.

• DELETE vs TRUNCATE vs DROP.

• Handling NULLs.

• Aggregate Functions.

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Day 7–11: SQL Patterns & Problem Practice

Day 7 — Pattern 1: Find Duplicates & Pattern 2: Top-N per Group

• LeetCode: Duplicate Emails

• DataLemur: Duplicate Job Listings (LinkedIn)

• LeetCode: Department Top Three Salaries

• DataLemur: Top Fans Rank (Spotify)

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Day 8 — Pattern 3: Detect Missing Data & Pattern 4: Second Highest Value

• LeetCode: Sales Analysis III (closest match for missing data)

• LeetCode: Second Highest Salary

• LeetCode: Nth Highest Salary

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Day 9 — Pattern 5: Pivot Data & Pattern 6: Running Totals

• DataLemur: Laptop vs. Mobile Viewership (NYTimes)

• DataLemur: Time Spent on Snaps (Snapchat)

• LeetCode: Game Play Analysis III

• DataLemur: Long Calls Growth (UnitedHealth)

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Day 10 — Pattern 7: Employee Hierarchy & Pattern 8: Complex Joins

• DataLemur: Senior Managers (Google)

• LeetCode: Trips and Users

• LeetCode: Customers Who Never Order

• LeetCode: Average Selling Price

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Day 11 — Pattern 9: Percentage Contribution & Pattern 10: Overlapping Ranges

• DataLemur: Time Spent on Snaps (Snapchat) (percent contributions)

• LeetCode: Average Selling Price (interval join)

• DataLemur: Total Utilization Time (AWS)

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Day 12–14: Revision & Mock Drills

Day 12 — Revise Theory (Q1–12).
Day 13 — Revise Theory (Q13–25).
Day 14 — Pattern Revision: mix practice across duplicates, top-N, joins, running totals.

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Day 15: Mock Interview Simulation

• 30 mins of theory Q&A (explain concepts aloud without SQL).

• 60 mins of problem solving: pick 2 LeetCode + 1 DataLemur problem.

• Reflect and revisit weak areas.