mysql里怎么打判断

SELECT 
    IF(salary > 5000, 'High Salary', 'Low Salary') AS salary_level
FROM 
    employees;

SELECT 
    CASE 
        WHEN salary > 10000 THEN 'Very High'
        WHEN salary > 5000 THEN 'High'
        WHEN salary > 2000 THEN 'Medium'
        ELSE 'Low'
    END AS salary_category
FROM 
    employees;

DELIMITER //

CREATE PROCEDURE CheckSalary(IN emp_id INT)
BEGIN
    DECLARE salary_level VARCHAR(20);
    
    SELECT salary INTO @salary FROM employees WHERE id = emp_id;
    
    IF @salary > 5000 THEN 
        SET salary_level = 'High Salary';
    ELSE 
        SET salary_level = 'Low Salary';
    END IF;
    
    SELECT salary_level;
END //

DELIMITER ;

DELIMITER //

CREATE TRIGGER before_employee_update
BEFORE UPDATE ON employees
FOR EACH ROW
BEGIN
    IF NEW.salary != OLD.salary THEN
        INSERT INTO salary_changes (employee_id, old_salary, new_salary, change_date)
        VALUES (OLD.id, OLD.salary, NEW.salary, NOW());
    END IF;
END //

DELIMITER ;