如何通过一条SQL语句获取计数值？

SELECT COUNT(*) FROM table_name;

SELECT COUNT(column_name) FROM table_name;

SELECT COUNT(*) FROM table_name WHERE condition;

SELECT column_name, COUNT(*) FROM table_name GROUP BY column_name;

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department VARCHAR(50),
    salary DECIMAL(10, 2)
);

SELECT COUNT(*) FROM employees;

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

SELECT COUNT(*) FROM employees WHERE salary > 50000;