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LeetCode SQL 50 Solution/1378. Replace Employee ID With The Unique Identifier/readme.md

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+Here’s a well-structured `README.md` for **LeetCode 1378 - Replace Employee ID With The Unique Identifier**, formatted for a GitHub repository:  
+
+```md
+# 🏢 Replace Employee ID With The Unique Identifier - LeetCode 1378
+
+## 📌 Problem Statement
+You are given two tables: **Employees** and **EmployeeUNI**.
+
+Your task is to return a table with:
+- Each employee's **unique ID** if it exists.
+- If an employee **does not** have a unique ID, return `NULL`.
+
+The result can be returned in **any order**.
+
+---
+
+## 📊 Table Structure
+
+### **Employees Table**
+| Column Name | Type    |
+| ----------- | ------- |
+| id          | int     |
+| name        | varchar |
+
+- `id` is the **primary key** (unique for each employee).
+- `name` is the **employee's name**.
+
+---
+
+### **EmployeeUNI Table**
+| Column Name | Type |
+| ----------- | ---- |
+| id          | int  |
+| unique_id   | int  |
+
+- `(id, unique_id)` is the **primary key** (ensuring unique mapping of employee IDs to unique IDs).
+- Each employee **may or may not** have a corresponding **unique ID**.
+
+---
+
+## 📊 Example 1:
+### **Input:**
+#### **Employees Table**
+| id  | name     |
+| --- | -------- |
+| 1   | Alice    |
+| 7   | Bob      |
+| 11  | Meir     |
+| 90  | Winston  |
+| 3   | Jonathan |
+
+#### **EmployeeUNI Table**
+| id  | unique_id |
+| --- | --------- |
+| 3   | 1         |
+| 11  | 2         |
+| 90  | 3         |
+
+### **Output:**
+| unique_id | name     |
+| --------- | -------- |
+| null      | Alice    |
+| null      | Bob      |
+| 2         | Meir     |
+| 3         | Winston  |
+| 1         | Jonathan |
+
+### **Explanation:**
+- `Alice` and `Bob` **do not have** a unique ID, so we return `NULL`.
+- The **unique ID** of `Meir` is **2**.
+- The **unique ID** of `Winston` is **3**.
+- The **unique ID** of `Jonathan` is **1**.
+
+---
+
+## 🖥 SQL Solution
+
+### ✅ **Using `LEFT JOIN`**
+#### **Explanation:**
+- Use a **LEFT JOIN** to **include all employees**.
+- If an employee **does not have** a matching `unique_id`, return `NULL`.
+
+```sql
+SELECT eu.unique_id, e.name
+FROM Employees e
+LEFT JOIN EmployeeUNI eu
+ON e.id = eu.id;
+```
+
+### ✅ **Using `USING(id)`**
+#### **Explanation:**
+- `USING(id)` is a cleaner alternative when both tables share a column.
+
+```sql
+SELECT unique_id, name
+FROM Employees
+LEFT JOIN EmployeeUNI
+USING (id);
+```
+
+### ✅ **Sorting by `id` (Optional)**
+#### **Explanation:**
+- If you want to return the result **sorted by `id`**, add `ORDER BY e.id`:
+
+```sql
+SELECT eu.unique_id, e.name
+FROM Employees e
+LEFT JOIN EmployeeUNI eu
+ON e.id = eu.id
+ORDER BY e.id;
+```
+
+---
+
+## 🐍 Pandas Solution (Python)
+#### **Explanation:**
+- Merge `Employees` with `EmployeeUNI` **using `left` join** on `id`.
+- Fill missing values (`NaN`) with `None`.
+
+```python
+import pandas as pd
+
+def replace_employee_id(employees: pd.DataFrame, employee_uni: pd.DataFrame) -> pd.DataFrame:
+    merged_df = employees.merge(employee_uni, on="id", how="left")
+    return merged_df[["unique_id", "name"]]
+```
+
+---
+
+## 📁 File Structure
+```
+📂 Replace-Employee-ID
+│── 📜 README.md
+│── 📜 solution.sql
+│── 📜 solution_pandas.py
+│── 📜 test_cases.sql
+```
+
+---
+
+## 🔗 Useful Links
+- 📖 [LeetCode Problem](https://leetcode.com/problems/replace-employee-id-with-the-unique-identifier/)
+- 📚 [SQL `LEFT JOIN`](https://www.w3schools.com/sql/sql_join_left.asp)
+- 🐍 [Pandas Merge Documentation](https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.merge.html)
+```
+
+### Features of this `README.md`:
+✅ **Clear problem description with tables**  
+✅ **Example with step-by-step explanation**  
+✅ **SQL and Pandas solutions with detailed breakdowns**  
+✅ **File structure for easy organization**  
+✅ **Helpful references for further learning**  
+
+Would you like any modifications? 🚀