mysql case取范围

SELECT 
    employee_id,
    salary,
    CASE 
        WHEN salary < 3000 THEN 'Low'
        WHEN salary BETWEEN 3000 AND 6000 THEN 'Medium'
        ELSE 'High'
    END AS salary_level
FROM employees;

SELECT 
    employee_id,
    salary,
    CASE 
        WHEN salary < 3000 THEN 'Low'
        WHEN salary >= 3000 AND salary <= 6000 THEN 'Medium'
        ELSE 'High'
    END AS salary_level
FROM employees;

SELECT 
    department_id,
    AVG(CASE WHEN salary < 3000 THEN 1 ELSE 0 END) AS low_salary_count,
    AVG(CASE WHEN salary BETWEEN 3000 AND 6000 THEN 1 ELSE 0 END) AS medium_salary_count,
    AVG(CASE WHEN salary > 6000 THEN 1 ELSE 0 END) AS high_salary_count
FROM employees
GROUP BY department_id;