Oracle别名（alias）是一种为表、列或者子查询分配临时名称的方法，它可以简化复杂查询中的表达式和提高可读性
SELECT emp_first_name AS first_name, emp_last_name AS last_name
FROM employees;
SELECT e.employee_id, d.department_id
FROM employees e, departments d
WHERE e.department_id = d.department_id;
SELECT employee_id, salary, salary * 12 AS annual_salary
FROM employees;
SELECT *
FROM (SELECT employee_id, salary FROM employees) AS subquery
WHERE subquery.salary > 5000;
SELECT e.employee_id, e.first_name, d.department_name
FROM employees e
JOIN departments d ON e.department_id = d.department_id;
SELECT department_id, COUNT(*) AS num_employees
FROM employees
GROUP BY department_id;
总之，在复杂查询中使用别名可以提高查询的可读性和易于理解，同时也有助于简化查询语句。
