Database Grouping and Sorting Methods

There are two methods for grouping and sorting in a database: sorting by fields and sorting by calculated expressions.

1. Grouping and sorting by fields: By using the GROUP BY clause, the result set can be grouped by specified fields, and the ORDER BY clause can be used to sort each group. For example, the following example groups the employee table by department and sorts the employees in each department in descending order of salary.

SELECT department, MAX(salary) AS max_salary
FROM employees
GROUP BY department
ORDER BY max_salary DESC;

1. Group and sort the result set by the calculation expression: Use the GROUP BY clause to group the result set by the calculation expression, and use the ORDER BY clause to sort each group. The calculation expression can be operations between fields, function calls, etc. For example, the following example groups the employee table based on the oddness or evenness of the year they joined the company and sorts each group by the number of employees.

SELECT CASE WHEN (YEAR(hire_date) % 2) = 0 THEN 'Even' ELSE 'Odd' END AS hire_year_group,
    COUNT(*) AS employee_count
FROM employees
GROUP BY hire_year_group
ORDER BY employee_count DESC;

Group sorting can be achieved by grouping the results using the GROUP BY clause and then sorting each group using the ORDER BY clause, regardless of the method used. This allows for sorting the results by a specified field or calculated expression.