This SQL query correctly addresses the user's request. The SELECT statement retrieves the cellular number, the carrier, and the sum of call durations. The SUM(callduration) is an aggregate function that calculates the total duration. When using an aggregate function, all other columns in the SELECT list that are not part of an aggregate function must be included in the GROUP BY clause. This query correctly groups the results by both cellularnumber and callednumbercarrier. Finally, the ORDER BY callednumbercarrier clause sorts the output as required.

A. The GROUP BY clause is incomplete. It must include all non-aggregated columns from the SELECT list, but it omits callednumbercarrier.

B. The GROUP BY clause incorrectly groups by callduration instead of the required cellularnumber, and it attempts to sort by a column not in the final output.

D. The GROUP BY clause is incorrect. It groups by callduration and omits the non-aggregated columns cellularnumber and callednumbercarrier from the SELECT list.

1. PostgreSQL 16 Documentation

Section 7.2.4. Grouping and Window Functions: "When GROUP BY is present

or if any aggregate functions are present

it is not valid for the SELECT list expressions to refer to ungrouped columns except within aggregate functions or when the ungrouped column is functionally dependent on the grouped columns

since there would otherwise be more than one possible value to return for an ungrouped column." This rule directly invalidates options A

B

and D

while confirming C is correct.

2. Stanford University

CS 145

Introduction to Databases

Lecture Notes on SQL Aggregation: The notes state the fundamental rule of SQL aggregation: "Attributes in SELECT list that are not aggregated must be in GROUP BY list." This principle is the basis for why option C is the only valid query among the choices.

3. MIT OpenCourseWare

6.830/6.814: Database Systems

Fall 2018

Lecture 2 Notes

Section 3.2 Aggregation: "The GROUP BY clause partitions the set of tuples into groups. The aggregate is then applied to each group... Any attribute that is not aggregated in the SELECT clause must appear in the GROUP BY clause." This academic source reinforces the standard SQL syntax and logic used in the correct answer.