单项选择题 In which case would you use a FULL OUTER JOIN?（）

单项选择题 You need to change the definition of an existing table. The COMMERCIALS table needs its DESCRIPTION column changed to hold varying length characters up to 1000 bytes. The column can currently hold 500 bytes per value. The table contains 20000 rows. Which statement is valid?（）

单项选择题 You need to modify the STUDENTS table to add a primary key on the STUDENT_ID column. The table is currently empty. Which statement accomplishes this task?（）