Add SQL GROUP BY Statement

docs/languages/SQL/sql-21.md

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+---
+id: sql-group-by
+sidebar_position: 21
+title: "SQL GROUP BY Statement"
+sidebar_label: "SQL GROUP BY"
+description: "The SQL GROUP BY statement is used to group rows that have the same values in specified columns and apply aggregate functions."
+tags: [sql, dbms, database, group by]
+---
+
+The `GROUP BY` statement in SQL is used to arrange identical data into groups. It is typically used with aggregate functions (such as `COUNT`, `SUM`, `AVG`, `MAX`, or `MIN`) to perform operations on each group.
+
+### Syntax
+
+```sql
+SELECT COUNT(CustomerID), Country
+FROM Customers
+GROUP BY Country
+HAVING COUNT(CustomerID) > 5;
+```
+
+### Key Points
+
+* The GROUP BY statement groups rows that have the same values in specified columns.
+* It is often combined with aggregate functions to perform calculations on each group.
+* Using ORDER BY with GROUP BY allows you to sort the grouped results.
+
+### Examples
+
+**Example 1: Count the number of products in each category**
+
+```sql
+SELECT CategoryID, COUNT(ProductID) AS ProductCount
+FROM Products
+GROUP BY CategoryID;
+```
+
+**Example 2: Find the total sales for each salesperson**
+
+```sql
+SELECT SalespersonID, SUM(SaleAmount) AS TotalSales
+FROM Sales
+GROUP BY SalespersonID;
+```
+
+**Example 3: Retrieve the average salary of employees in each department**
+
+```sql
+SELECT DepartmentID, AVG(Salary) AS AverageSalary
+FROM Employees
+GROUP BY DepartmentID;
+```
+
+**Note:**
+* The GROUP BY clause must appear after the WHERE clause and before the ORDER BY clause if they are used together.
+* When using GROUP BY, only the grouped columns or aggregate functions can be included in the SELECT statement, as other columns would produce ambiguous results.
+* The HAVING clause can be used with GROUP BY to filter groups based on aggregate conditions.