SQL Grouping — GROUP BY & HAVING | Learn Computer Academy

Grouping — GROUP BY & HAVING

GROUP BY collapses rows into summarized groups, and HAVING filters those groups after they're formed.

GROUP BY

Groups rows that share a value so aggregate functions can summarize each group.

SELECT column, AGG(column)
FROM table_name
GROUP BY column;

SELECT department, COUNT(*)
FROM Employee
GROUP BY department;

department	count
IT	12
HR	5

Common Mistake

Selecting a column that's neither grouped nor wrapped in an aggregate function — most engines reject this outright.

Filters grouped results, the way WHERE filters individual rows.

SELECT column, AGG(column)
FROM table_name
GROUP BY column
HAVING condition;

SELECT department, COUNT(*)
FROM Employee
GROUP BY department
HAVING COUNT(*) > 5;

Filters groups after GROUP BY has formed them.
Runs after grouping, whereas WHERE runs before — that ordering is the whole reason HAVING exists.

Common Mistake

Using WHERE to filter on an aggregate value — that's HAVING's job specifically, because the aggregate doesn't exist yet at the point WHERE runs.

Grouping and filtering set up the last piece of core querying: combining data from more than one table at once, with joins.