How Does the SQL `GROUP BY` Clause Aggregate Data and When Does it Return a Single Row?

SQL's GROUP BY Clause: Data Aggregation and Row Output

Data aggregation is a cornerstone of database analysis. SQL's GROUP BY clause provides the mechanism for grouping rows based on specified columns and applying aggregate functions to those groups.

Let's examine a query using a sample table, Tab1, with columns a1, a2, a3, etc. (none uniquely identifying a row). Consider this query:

SELECT a1, a2, SUM(a3) FROM Tab1 GROUP BY a1, a2;

Deciphering the Query's Results

The GROUP BY clause divides Tab1's rows into groups based on unique combinations of a1 and a2. The query produces one row for each distinct (a1, a2) pair.

For every group, the SUM(a3) function calculates the total of a3 values. Thus, the output shows the aggregated a3 sum for each unique (a1, a2) combination.

Will it Always Return a Single Row?

The query's output isn't always a single row. It returns multiple rows—one per unique (a1, a2) group. The number of rows depends directly on the number of distinct groups in Tab1. Only if Tab1 has one row, or if all rows share identical a1 and a2 values, would the query produce a single row.

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