📊 SQL Learning Journey — Week 4 Recap

🧠 Key Concepts Explored

🧪 Subqueries Simplified

Subqueries are powerful tools in SQL. This week, I learned that:

• Subquery is independent — it returns a value.
• Outer query uses that value, applying it across the main table.

💡 Golden Rule for Subqueries:

Don't try to understand the whole table first. Instead, zoom in:

1. Who should be in the result?
2. What is the condition to qualify?
3. What’s the minimal data I need?

🎯 3-Step SQL Problem Solving Template:

Step	Question	Example Thought
1	Who is the output about?	Authors
2	What must they have done?	Viewed their own article
3	What’s the SQL logic?	author_id = viewer_id → SELECT DISTINCT author_id

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🧰 Handy Functions

Function	Purpose	Syntax Example	Result
DATE_ADD()	Adds interval to date	DATE_ADD('2025-08-06', INTERVAL 1 DAY)	2025-08-07
DATE_SUB()	Subtracts interval from date	DATE_SUB('2025-08-06', INTERVAL 1 DAY)	2025-08-05

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🧾 ENUM Usage

ENUM is a data type used to restrict column values to a predefined set.

Example:

status ENUM('Pending', 'Shipped', 'Delivered', 'Cancelled')

• Only those 4 values are allowed.
• You can include numbers, but they’re stored as strings, not integers.

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✅ LeetCode Practice — 10 Problems Solved 🎉

This week, I tackled and completed 10 beginner-friendly SQL problems:

Problem No.	Title	Difficulty
1757	Recyclable and Low Fat Products	Easy
584	Find Customer Referee	Easy
595	Big Countries	Easy
1148	Article Views I	Easy
1683	Invalid Tweets	Easy
1378	Replace Employee ID With The Unique Identifier	Easy
1068	Product Sales Analysis I	Easy
1581	Customer Who Visited but Did Not Make Any Transactions	Easy
197	Rising Temperature	Easy
1661	Average Time of Process per Machine	Easy
577	Employee Bonus	Easy

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🌱 What’s Next?

• Dive deeper into Window Functions
• Explore Advanced Subqueries and Nested Joins
• Build a mini-project using real-world datasets

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Thanks for reading! Feel free to connect and share your learning journey too. 💬

#SQL #100DaysOfSQL #LearningJourney #Subqueries #LeetCode