Update readme.md

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LeetCode SQL 50 Solution/1141. User Activity for the Past 30 Days I/readme.md

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+Here’s a well-structured `README.md` file for **LeetCode 1141 - User Activity for the Past 30 Days I** with SQL and Pandas solutions:
+
+```md
+# 📊 User Activity for the Past 30 Days I - LeetCode 1141
+
+## 📌 Problem Statement
+You are given the **Activity** table that records user activities on a social media website.
+
+### Activity Table
+| Column Name | Type |
+| ------------- | ---- |
+| user_id | int |
+| session_id | int |
+| activity_date | date |
+| activity_type | enum |
+
+- The `activity_type` column is an ENUM of **('open_session', 'end_session', 'scroll_down', 'send_message')**.
+- Each session belongs to exactly **one user**.
+- The table **may have duplicate rows**.
+
+### Task:
+Find the **daily active user count** for a period of **30 days ending 2019-07-27 inclusively**.
+- A user is considered **active on a given day** if they made at least **one activity**.
+- Ignore days with **zero active users**.
+
+---
+
+## 📊 Example 1:
+### Input:
+**Activity Table**
+| user_id | session_id | activity_date | activity_type |
+| ------- | ---------- | ------------- | ------------- |
+| 1 | 1 | 2019-07-20 | open_session |
+| 1 | 1 | 2019-07-20 | scroll_down |
+| 1 | 1 | 2019-07-20 | end_session |
+| 2 | 4 | 2019-07-20 | open_session |
+| 2 | 4 | 2019-07-21 | send_message |
+| 2 | 4 | 2019-07-21 | end_session |
+| 3 | 2 | 2019-07-21 | open_session |
+| 3 | 2 | 2019-07-21 | send_message |
+| 3 | 2 | 2019-07-21 | end_session |
+| 4 | 3 | 2019-06-25 | open_session |
+| 4 | 3 | 2019-06-25 | end_session |
+
+### Output:
+| day | active_users |
+| ---------- | ------------ |
+| 2019-07-20 | 2 |
+| 2019-07-21 | 2 |
+
+### Explanation:
+- **2019-07-20**: Users **1 and 2** were active.
+- **2019-07-21**: Users **2 and 3** were active.
+- **Days with zero active users are ignored**.
+
+---
+
+## 🖥 SQL Solutions
+
+### 1️⃣ Standard MySQL Solution
+#### Explanation:
+- **Filter records** for the last **30 days** (ending on `2019-07-27`).
+- Use `COUNT(DISTINCT user_id)` to count **unique active users per day**.
+- Ignore **days with zero active users**.
+
+```sql
+SELECT 
+ activity_date AS day, 
+ COUNT(DISTINCT user_id) AS active_users
+FROM 
+ Activity
+WHERE 
+ DATEDIFF('2019-07-27', activity_date) < 30 
+ AND DATEDIFF('2019-07-27', activity_date) >= 0
+GROUP BY activity_date;
+```
+
+---
+
+### 2️⃣ Alternative Solution Using `BETWEEN`
+#### Explanation:
+- This solution filters the date range using `BETWEEN` instead of `DATEDIFF`.
+
+```sql
+SELECT 
+ activity_date AS day, 
+ COUNT(DISTINCT user_id) AS active_users
+FROM 
+ Activity
+WHERE 
+ activity_date BETWEEN DATE_SUB('2019-07-27', INTERVAL 29 DAY) AND '2019-07-27'
+GROUP BY activity_date;
+```
+
+---
+
+## 🐍 Pandas Solution (Python)
+#### Explanation:
+- Filter activity records for the **last 30 days**.
+- **Group by `activity_date`** and count **unique `user_id`s**.
+- **Ignore days with zero active users**.
+
+```python
+import pandas as pd
+
+def daily_active_users(activity: pd.DataFrame) -> pd.DataFrame:
+ # Filter data within the last 30 days (ending on '2019-07-27')
+ filtered = activity[(activity["activity_date"] >= "2019-06-28") & (activity["activity_date"] <= "2019-07-27")]
+
+ # Group by day and count unique users
+ result = filtered.groupby("activity_date")["user_id"].nunique().reset_index()
+
+ # Rename columns
+ result.columns = ["day", "active_users"]
+ return result
+```
+
+---
+
+## 📁 File Structure
+```
+📂 User-Activity-Past-30-Days
+│── 📜 README.md
+│── 📜 solution.sql
+│── 📜 solution_between.sql
+│── 📜 solution_pandas.py
+│── 📜 test_cases.sql
+```
+
+---
+
+## 🔗 Useful Links
+- 📖 [LeetCode Problem](https://leetcode.com/problems/user-activity-for-the-past-30-days-i/)
+- 📚 [SQL Date Functions](https://www.w3schools.com/sql/sql_dates.asp)
+- 🐍 [Pandas Documentation](https://pandas.pydata.org/docs/)
+```
+
+This README includes:
+- **Clear problem statement**
+- **Input and output tables**
+- **Multiple SQL solutions with explanations**
+- **Pandas solution in Python**
+- **File structure for a clean GitHub repo**
+- **Useful links**
+
+Let me know if you need any changes! 🚀