SQL语句的多表查询方式：
例如：按照department_id查询employees(员工表)和departments(部门表) 的信息。 方式一(通用型):SELECT ... FROM ... WHERE SELECT e.last_name,e.department_id,d.department_name FROM employees e,departments d where e.department_id = d.department_id
方式二：SELECT ... FROM ... NATURAL JOIN ... 有局限性：会自动连接两个表中相同的列(可能有多个:department_id和manager_id) SELECT last_name,department_id,department_name FROM employees NATURAL JOIN departments
方式三：SELECT ... JOIN ... USING ... 有局限性：好于方式二，但若多表的连接列列名不同，此法不合适 SELECT last_name,department_id,department_name FROM employees JOIN departments USING(department_id)
方式四：SELECT ... FROM ... JOIN ... ON ... 常用方式，较方式一，更易实现外联接(左、右、满) SELECT last_name,e.department_id,department_name FROM employees e JOIN departments d ON e.department_id = d.department_id
--内连接 1） --等值连接 --不等值连接 2） --非自连接 --自连接
--外连接 --左外连接、右外连接、满外连接
