Article Introduction：union: Perform a union operation on multiple result sets, excluding duplicate rows, and sort them at the same time. unionall: Performs a union operation on multiple result sets, including duplicate rows, without sorting. Query the information of employees whose department is less than 30, and the information of employees whose department is greater than 20 and less than 40. ①. First query the information of employees whose department number is less than 30. SELECTemployees_id,last_name,salary,department_idFROMemployeesWHEREdepartment_id

2023-05-30comment 0870