单项选择题 Which of the following statements eliminates all but one of each set of duplicate rows in the DEPT column in the STAFF table?（）

单项选择题 In which of the following situations should correlation names be used?（）

单项选择题 Consider the following table called EMPLOYEES: ID FIRSTNAME LASTNAME JOB LEVEL CLERK 3 If the following SQL statement is executed, how many rows will be deleted? DELETE FROM employees WHERE 1 = 1（）