MySQL COUNT()、SUM() 和 GROUP BY

MySQL COUNT()、SUM() 和 GROUP BY 实现数据统计和分组

sql
SELECT COUNT(*) FROM table_name;

sql
SELECT SUM(column_name) FROM table_name;

sql
SELECT department, COUNT(*) FROM table_name GROUP BY department;

sql
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    department VARCHAR(50),
    salary DECIMAL(10, 2)
);
INSERT INTO employees (id, name, department, salary) VALUES
(1, 'John', 'HR', 5000),
(2, 'Jane', 'HR', 6000),
(3, 'Mike', 'IT', 7000),
(4, 'Emily', 'IT', 8000),
(5, 'Tom', 'Finance', 9000),
(6, 'Lucy', 'Finance', 10000);
SELECT department, COUNT(*) AS employee_count, AVG(salary) AS average_salary
FROM employees
GROUP BY department;