31.choose the best answer
Which statement is true regarding the USING clause in table joins?
A) It can be used to access data from tables through equijoins as well as nonequijoins.
B) It can be used to join tables that have columns with the same name and compatible data types.
C) It can be used to join a maximum of three tables.
D) It can be used to restrict the number of columns used in a NATURAL join.
Answer：B
(解析:using 只能用在 equiue join ，而且必须列的名字要相同，类型可以不同，但是可以隐式转过去)
