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Author: iamAntimPal

LeetCode SQL 50 Solution/180. Consecutive Numbers/readme.md

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+# 180. Consecutive Numbers
+
+## Problem Statement
+Table: `Logs`
+
+| Column Name | Type    |
+| ----------- | ------- |
+| id          | int     |
+| num         | varchar |
+
+- `id` is the primary key (auto-incremented starting from 1).
+- Find all numbers that appear **at least three times consecutively**.
+- Return the result table in **any order**.
+
+### Example 1:
+
+#### Input:
+| id  | num |
+| --- | --- |
+| 1   | 1   |
+| 2   | 1   |
+| 3   | 1   |
+| 4   | 2   |
+| 5   | 1   |
+| 6   | 2   |
+| 7   | 2   |
+
+#### Output:
+| ConsecutiveNums |
+| --------------- |
+| 1               |
+
+---
+
+## Solution
+
+```sql
+SELECT DISTINCT num AS ConsecutiveNums
+FROM Logs l1
+JOIN Logs l2 ON l1.id = l2.id - 1
+JOIN Logs l3 ON l2.id = l3.id - 1
+WHERE l1.num = l2.num AND l2.num = l3.num;
+```
+
+### Explanation:
+- We use **self-joins** to check three consecutive rows where `num` values are the same.
+- `l1`, `l2`, and `l3` represent three consecutive rows.
+- The condition `l1.num = l2.num AND l2.num = l3.num` ensures that we only select numbers appearing at least three times consecutively.
+- `DISTINCT` ensures we don't get duplicate results.
+
+---
+
+## Alternative Approach using `LAG()` (MySQL 8+)
+
+```sql
+WITH Consecutive AS (
+    SELECT num,
+           LAG(num, 1) OVER (ORDER BY id) AS prev1,
+           LAG(num, 2) OVER (ORDER BY id) AS prev2
+    FROM Logs
+)
+SELECT DISTINCT num AS ConsecutiveNums
+FROM Consecutive
+WHERE num = prev1 AND num = prev2;
+```
+
+### Explanation:
+- We use the `LAG()` function to check the previous two rows for the same `num` value.
+- If a `num` matches with its two previous values, it qualifies as a **consecutive number appearing at least three times**.
+
+---
+
+## File Structure
+
+```
+📂 ConsecutiveNumbers
+ ├── 📄 README.md  # Problem statement, approach, and solutions
+ ├── 📄 consecutive_numbers.sql  # SQL solution
+ ├── 📄 alternative_solution.sql  # Alternative solution using LAG()
+```
+
+---
+
+## Useful Links
+- [LeetCode Problem](https://leetcode.com/problems/consecutive-numbers/) 🚀
+- [SQL `JOIN` Explained](https://www.w3schools.com/sql/sql_join.asp)
+- [MySQL `LAG()` Window Function](https://dev.mysql.com/doc/refman/8.0/en/window-function-descriptions.html)
+
+---
+
+Feel free to contribute with optimized solutions! 💡