Aggregation functions perform a calculation on a set of values and return a single value. GROUP BY is used to arrange identical data into groups.

• COUNT(): Returns the number of rows.
• SUM(): Returns the total sum of a numeric column.
• AVG(): Returns the average value.
• MIN() / MAX(): Returns the smallest/largest value.

This is the most common aggregation question:

• WHERE: Filters rows BEFORE grouping. It cannot be used with aggregate functions (like SUM).
• HAVING: Filters groups AFTER grouping. It is used specifically with aggregate functions.

sql Standard
SELECT 
    Department, 
    COUNT(*) as EmployeeCount,
    AVG(Salary) as AvgSalary
FROM Employees
WHERE Status = 'Active' -- Filter rows first
GROUP BY Department
HAVING AVG(Salary) > 50000; -- Filter groups after

• Non-aggregated columns: Including a column in the SELECT list that is NOT part of the GROUP BY clause (this is an error in most SQL engines, although MySQL allows it under some settings).
• ORDER BY location: Placing ORDER BY before GROUP BY or HAVING (it must always come last).
• NULLs in aggregates: Forgetting that aggregate functions (except COUNT(*)) ignore NULL values.