GROUP BY, aggregates, and HAVING vs WHERE

How grouping collapses rows, and the exact reason HAVING exists as a separate clause.

12 min read · Lesson 4 of 6 · Pro

Aggregates collapse many rows into one

An aggregate function reads a pile of rows and returns a single value. There are five you need.

COUNT(*) — how many rows
SUM(col) — total
AVG(col) — average
MIN(col) / MAX(col) — smallest and largest

SQL

SELECT COUNT(*) AS total_students, AVG(cgpa) AS avg_cgpa
FROM students;

One row out, no matter how many rows went in. That is the defining property.

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