mysql的Join算法

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  `emp_no` int(11) NOT NULL,

  CONSTRAINT `dept_emp_ibfk_1` FOREIGN KEY (`emp_no`) REFERENCES `employees` (`emp_no`) ON DELETE CASCADE

CREATE TABLE `dept_emp` (

  PRIMARY KEY (`emp_no`,`dept_no`),

) ENGINE=InnoDB DEFAULT CHARSET=lati

  KEY `from_date` (`from_date`),

  `dept_no` char(4) NOT NULL,

  `to_date` date NOT NULL,

  `from_date` date NOT NULL,