How Does the SQL `GROUP BY` Clause Aggregate and Summarize Data?

Mastering SQL's GROUP BY Clause for Data Aggregation

The GROUP BY clause in SQL is a fundamental tool for summarizing and aggregating data within a table. This guide clarifies its function for database users and developers.

Imagine a table, Tab1, containing columns a1, a2, a3, etc., where values aren't necessarily unique. Executing a query like this:

<code class="language-sql">SELECT a1, a2, SUM(a3) FROM Tab1 GROUP BY a1, a2</code>

instructs the database to group rows in Tab1 based on unique pairings of a1 and a2 values. Rows sharing identical a1 and a2 values are grouped together.

For each group, the SUM(a3) aggregate function computes the sum of a3 values across all rows within that group. The output presents these summarized groups as individual rows.

Understanding the Query's Output

The query yields one row for every unique combination of a1 and a2 found in Tab1. This is the core function of GROUP BY: representing each distinct data grouping with a single row.

For instance, given this Tab1 data:

a1	a2	a3
1	A	10
1	A	20
2	B	30
2	B	40

The query's output would be:

a1	a2	SUM(a3)
1	A	30
2	B	70

This demonstrates how GROUP BY groups rows based on a1 and a2, independently calculating the sum of a3 for each group.

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