单项选择题
A.Both tables have NULL values.
B.You want all unmatched data from one table.
C.You want all matched data from both tables.
D.You want all unmatched data from both tables.
E.One of the tables has more data than the other.
F.You want all matched and unmatched data from only one table.
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?（）
多项选择题 Which two statements about subqueries are true? （）