How Does the GROUP BY Clause Aggregate Data in SQL?

Understanding SQL's GROUP BY Clause for Data Aggregation

The GROUP BY clause is fundamental to data summarization in SQL databases. Let's illustrate its function with a table named Tab1, containing attributes a1, a2, and others.

Consider this query:

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

Query Explanation:

This SQL statement groups Tab1's rows according to the values in a1 and a2. For each unique pairing of a1 and a2 values, it calculates the sum of the corresponding a3 values within that group.

Result Set Characteristics:

The query's output is a new table with:

• Columns: The original attributes a1 and a2, along with the aggregated SUM(a3) column.
• Rows: Each row represents a distinct combination of a1 and a2 from the original Tab1 table.

Multiple Rows Possible:

It's important to note that the GROUP BY clause doesn't inherently guarantee a single-row result. Multiple rows will be returned if Tab1 contains various unique combinations of a1 and a2 values. However, each resulting row will uniquely represent a group sharing identical a1 and a2 values.

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