SQL. SELECT

GROUP BY clause

The GROUP BY clause is used to group rows that have the same values in specified columns into summary rows. It is often used in conjunction with aggregate functions such as COUNT, SUM, AVG, MAX, AND MIN to perform operations on each group of rows.

Purpose:

• Grouping Data: It allows you to group rows that share a common value in one or more columns.
• Aggregating Data: It enables you to perform aggregate calculation on each group, such as summing values or counting rows.

SELECT column1, aggregate_function(column2)
FROM table_name
GROUP BY column1;

Example Consider a table named Order with the following columns:

• OrderID: unique identifier for each order
• CustomerID: Identifier for the customer who placed the order
• OrderAmount: The amount of the order

To find the total order amount for each customer, you can used the GROUP BY clause with the SUM aggregate function:

SELECT CustomerId, SUM(OrderAmount) As TotalOrderAmount
FROM Orders
GROUP BY CustomerId;

Explanation of GROUP BY CustomerId: Groups the results by CustomerId, so the SUM function is applied to each group of the rows with the same CustomerID.

The GROUP BY clause is essential for summarizing data and generating reports in SQL.