Miks agregaatfunktsioonidega päringute tulemuste väärtuste arv ei ole võrdne ridade arvuga?

The number of values in the query results with aggregate functions is not equal to the number of rows in the original table because aggregate functions (such as SUM, COUNT, AVG, MAX, MIN) reduce a set of rows to one or more values, summarizing the data. If an aggregate function is used in the query without GROUP BY, the result will be a single row with the aggregated value for the entire dataset.

If GROUP BY is used, then the number of rows in the result will be equal to the number of unique groups, not the number of original rows. For example, if grouping by a column with 5 unique values, the result will contain 5 rows — one for each group.

Example:

SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;

Here, the result contains one row for each department, not for each employee.