GROUP BY & HAVING Clauses

The GROUP BY clause is used to group rows that have the same value in one or more columns. It’s commonly used with aggregate functions like COUNT, SUM, or AVG to get summary data for each group.

Using GROUP BY

To group students by age and count how many students are in each group:

SELECT age, COUNT(*) FROM students GROUP BY age;

This returns one row per age, showing how many students have that age.

Filtering Groups with HAVING

While WHERE filters individual rows before grouping, the HAVING clause filters groups after aggregation. For example:

SELECT age, COUNT() FROM students GROUP BY age HAVING COUNT() > 1;

This shows only ages that appear more than once in the table.

‍