关于字段别名的问题

为啥将select后的字段起上别名后，HAVING中即能直接使用字段，也能使用别名，而where后面就不行呢？
例：SELECT AVG(salary) a,department_id
    FROM employees
    GROUP BY department_id
    HAVING AVG(salary)>1000;

SELECT AVG(salary) a,department_id
    FROM employees
    GROUP BY department_id
    HAVING a>1000;
两者都行

SELECT employee_id i,salary FROM employees
WHERE i>10;
这样的就不行