The role of group by in sql

The GROUP BY clause in SQL is used to group data and calculate aggregate values: group data by a specified column or expression. Calculate aggregate values ​​(such as SUM, COUNT, MIN, MAX, etc.) for each group. Reduces data set size, making it easier to process and analyze.

GROUP BY Purpose

The GROUP BY clause in SQL is used to divide the data set into specified columns or expressions Group by formula and calculate aggregate values ​​(such as SUM, COUNT, MIN, MAX, etc.) based on each group.

Function

The main functions of GROUP BY are as follows:

• Aggregation of data: Aggregation of data by group , generate summary information. For example, calculate the total order count for each customer or the total number of employees in each department.
• Group rows: Group data by a specified column or expression, combining rows with the same group value. This makes it easier to identify and analyze patterns and differences between different groups.
• Reduce data set size: Through grouping and aggregation, GROUP BY can significantly reduce the size of the data set, making it easier to process and analyze.

Syntax

The syntax of the GROUP BY clause is as follows:

<code>SELECT aggregation_function(column_name)
FROM table_name
GROUP BY column_name;</code>

Example

For example, the following query groups by customer and calculates the order quantity for each customer:

<code>SELECT COUNT(*), customer_name
FROM orders
GROUP BY customer_name;</code>

This query will return a result set where each row represents a unique customer, and their order quantity.

Note

• The GROUP BY clause must appear before the aggregate function.
• You can only group by columns or expressions referenced in the SELECT list or HAVING clause.
• If a column included in the GROUP BY clause contains a NULL value, a separate group is created within the grouping.

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