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| 1 | +Below is a well-structured `README.md` for **LeetCode 2356 - Number of Unique Subjects Taught by Each Teacher**, which includes both the SQL solution and a Python (Pandas) solution. |
| 2 | + |
| 3 | +```md |
| 4 | +# 📚 Number of Unique Subjects Taught by Each Teacher - LeetCode 2356 |
| 5 | + |
| 6 | +## 📌 Problem Statement |
| 7 | +You are given a table **Teacher** that provides information about the subjects taught by teachers in various departments of a university. |
| 8 | + |
| 9 | +Your task is to calculate the **number of unique subjects** each teacher teaches. |
| 10 | +Note that the table can have multiple rows for the same subject taught in different departments, but you should count each subject only once per teacher. |
| 11 | + |
| 12 | +Return the result table in **any order**. |
| 13 | + |
| 14 | +--- |
| 15 | + |
| 16 | +## 📊 Table Structure |
| 17 | + |
| 18 | +### **Teacher Table** |
| 19 | +| Column Name | Type | |
| 20 | +| ----------- | ---- | |
| 21 | +| teacher_id | int | |
| 22 | +| subject_id | int | |
| 23 | +| dept_id | int | |
| 24 | + |
| 25 | +- `(subject_id, dept_id)` is the **primary key**. |
| 26 | +- Each row indicates that the teacher with `teacher_id` teaches the subject `subject_id` in the department `dept_id`. |
| 27 | + |
| 28 | +--- |
| 29 | + |
| 30 | +## 📊 Example 1: |
| 31 | + |
| 32 | +### **Input:** |
| 33 | +#### **Teacher Table** |
| 34 | +| teacher_id | subject_id | dept_id | |
| 35 | +| ---------- | ---------- | ------- | |
| 36 | +| 1 | 2 | 3 | |
| 37 | +| 1 | 2 | 4 | |
| 38 | +| 1 | 3 | 3 | |
| 39 | +| 2 | 1 | 1 | |
| 40 | +| 2 | 2 | 1 | |
| 41 | +| 2 | 3 | 1 | |
| 42 | +| 2 | 4 | 1 | |
| 43 | + |
| 44 | +### **Output:** |
| 45 | +| teacher_id | cnt | |
| 46 | +| ---------- | --- | |
| 47 | +| 1 | 2 | |
| 48 | +| 2 | 4 | |
| 49 | + |
| 50 | +### **Explanation:** |
| 51 | +- **Teacher 1:** |
| 52 | + - Teaches subject **2** (in departments 3 and 4) and subject **3** (in department 3). |
| 53 | + - Unique subjects = {2, 3} → **2 subjects**. |
| 54 | +- **Teacher 2:** |
| 55 | + - Teaches subjects **1**, **2**, **3**, and **4** (all in department 1). |
| 56 | + - Unique subjects = {1, 2, 3, 4} → **4 subjects**. |
| 57 | + |
| 58 | +--- |
| 59 | + |
| 60 | +## 🖥 SQL Solution |
| 61 | + |
| 62 | +### ✅ **Approach:** |
| 63 | +- Use `COUNT(DISTINCT subject_id)` to count the number of unique subjects taught by each teacher. |
| 64 | +- Group the results by `teacher_id`. |
| 65 | + |
| 66 | +```sql |
| 67 | +SELECT teacher_id, COUNT(DISTINCT subject_id) AS cnt |
| 68 | +FROM Teacher |
| 69 | +GROUP BY teacher_id; |
| 70 | +``` |
| 71 | + |
| 72 | +--- |
| 73 | + |
| 74 | +## 🐍 Python (Pandas) Solution |
| 75 | + |
| 76 | +### ✅ **Approach:** |
| 77 | +1. **Group by `teacher_id`:** |
| 78 | + - Group the DataFrame by `teacher_id`. |
| 79 | +2. **Count Unique Subjects:** |
| 80 | + - Use the `nunique()` function on the `subject_id` column within each group to count unique subjects. |
| 81 | +3. **Reset Index and Rename:** |
| 82 | + - Reset the index and rename the column appropriately. |
| 83 | + |
| 84 | +```python |
| 85 | +import pandas as pd |
| 86 | + |
| 87 | +def count_unique_subjects(teacher: pd.DataFrame) -> pd.DataFrame: |
| 88 | + # Group by teacher_id and count unique subject_id values |
| 89 | + result = teacher.groupby('teacher_id')['subject_id'].nunique().reset_index() |
| 90 | + result = result.rename(columns={'subject_id': 'cnt'}) |
| 91 | + return result |
| 92 | + |
| 93 | +# Example usage: |
| 94 | +# teacher_df = pd.DataFrame({ |
| 95 | +# 'teacher_id': [1, 1, 1, 2, 2, 2, 2], |
| 96 | +# 'subject_id': [2, 2, 3, 1, 2, 3, 4], |
| 97 | +# 'dept_id': [3, 4, 3, 1, 1, 1, 1] |
| 98 | +# }) |
| 99 | +# print(count_unique_subjects(teacher_df)) |
| 100 | +``` |
| 101 | + |
| 102 | +--- |
| 103 | + |
| 104 | +## 📁 File Structure |
| 105 | +``` |
| 106 | +📂 Unique-Subjects-Per-Teacher |
| 107 | +│── README.md |
| 108 | +│── solution.sql |
| 109 | +│── solution_pandas.py |
| 110 | +│── test_cases.sql |
| 111 | +│── sample_data.csv |
| 112 | +``` |
| 113 | + |
| 114 | +--- |
| 115 | + |
| 116 | +## 🔗 Useful Links |
| 117 | +- 📖 [LeetCode Problem](https://leetcode.com/problems/number-of-unique-subjects-taught-by-each-teacher/) |
| 118 | +- 🔍 [MySQL COUNT(DISTINCT) Documentation](https://www.w3schools.com/sql/sql_count_distinct.asp) |
| 119 | +- 🐍 [Pandas GroupBy Documentation](https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.groupby.html) |
| 120 | +``` |
| 121 | + |
| 122 | +This `README.md` provides: |
| 123 | +- A **clear problem statement** along with the table structure. |
| 124 | +- A **detailed example** with explanation. |
| 125 | +- Both **SQL and Python (Pandas) solutions** with step-by-step approaches. |
| 126 | +- An **organized file structure** and helpful external links. |
| 127 | + |
| 128 | +Let me know if you need any modifications! |
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