Create grouping-data.md

Created a grouping-data.md file which contains the basic grouping syntax, along with where and having, and various aggregate functions.

Author: khushi-joshi-05

docs/sql/SQL-basics/grouping-data.md

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+# Grouping Data in SQL
+
+When you work with a lot of data, you often want to **combine rows that have the same values** in certain columns and calculate something for each group.  
+In SQL, this is done with the **`GROUP BY`** clause.
+
+
+
+## Why Use GROUP BY?
+
+Imagine you have a `sales` table:
+
+| id | product   | category  | quantity | price |
+|----|-----------|-----------|----------|-------|
+| 1  | Apple     | Fruit     | 10       | 2.5   |
+| 2  | Orange    | Fruit     | 5        | 3.0   |
+| 3  | Carrot    | Vegetable | 7        | 1.5   |
+| 4  | Apple     | Fruit     | 8        | 2.5   |
+
+If you want **total quantity sold for each category**, you can group the rows by `category`.
+
+
+
+## GROUP BY Syntax
+
+```sql
+SELECT column1, column2, ..., aggregate_function(column)
+FROM table_name
+GROUP BY column1, column2, ...;
+````
+
+* **`aggregate_function`**: Functions that calculate a value for a group, such as:
+
+  * `COUNT()` → Counts rows
+  * `SUM()` → Adds values
+  * `AVG()` → Calculates average
+  * `MIN()` → Finds smallest value
+  * `MAX()` → Finds largest value
+
+
+## Example: GROUP BY with SUM
+
+```sql
+SELECT category, SUM(quantity) AS total_quantity
+FROM sales
+GROUP BY category;
+```
+
+**Result:**
+
+| category  | total\_quantity |
+| --------- | --------------- |
+| Fruit     | 23              |
+| Vegetable | 7               |
+
+**How it works:**
+
+1. SQL looks at the `category` column.
+2. Rows with the same category are grouped together.
+3. The `SUM(quantity)` is calculated for each group.
+
+
+
+## GROUP BY with Multiple Columns
+
+You can group by **more than one column**.
+
+```sql
+SELECT category, product, SUM(quantity) AS total_quantity
+FROM sales
+GROUP BY category, product;
+```
+
+Now each unique **(category, product)** pair is its own group.
+
+
+
+## Filtering Groups with HAVING
+
+`WHERE` filters **rows before grouping**.
+`HAVING` filters **groups after grouping**.
+
+Example: Show only categories where total quantity > 10.
+
+```sql
+SELECT category, SUM(quantity) AS total_quantity
+FROM sales
+GROUP BY category
+HAVING SUM(quantity) > 10;
+```
+
+**Result:**
+
+| category | total\_quantity |
+| -------- | --------------- |
+| Fruit    | 23              |
+
+
+
+## Difference Between WHERE and HAVING
+
+| Clause | Filters On      | Works With Aggregates? |
+| ------ | --------------- | ---------------------- |
+| WHERE  | Individual rows | ❌ (no aggregates)      |
+| HAVING | Grouped results | ✅ (with aggregates)    |
+
+
+
+## Common Aggregate Functions
+
+| Function    | Description       | Example         |
+| ----------- | ----------------- | --------------- |
+| COUNT(\*)   | Counts all rows   | `COUNT(*)`      |
+| SUM(column) | Adds all values   | `SUM(quantity)` |
+| AVG(column) | Average of values | `AVG(price)`    |
+| MIN(column) | Minimum value     | `MIN(price)`    |
+| MAX(column) | Maximum value     | `MAX(price)`    |
+