How Does SQL's GROUP BY Clause Transform Data with Non-Unique Attributes?

Understanding SQL's GROUP BY Clause with Non-Unique Data

The GROUP BY clause is essential for transforming data in SQL tables, particularly when dealing with non-unique attributes. Let's illustrate this using a sample table, Tab1, containing non-unique attribute values.

The following SQL query uses GROUP BY for data aggregation:

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

Query Functionality Explained

This query groups the data based on unique combinations of a1 and a2. For each distinct pair of a1 and a2 values:

• It creates a group containing all rows with matching attribute values.
• It sums the a3 values within each group.
• The result is a single row per group, showing the a1, a2 values, and the calculated sum of a3.

Therefore, the output won't be a single row, but rather multiple rows, each representing a unique combination of a1 and a2 found in the table.

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