121. Which two statements are true regarding the USING clause in table joins? (Choose two .)
A. It can be used to join a maximum of three tables.
B. It can be used to restrict the number of columns used in a NATURAL join.
C. It can be used to access data from tables through equijoins as well as nonequijoins.
D. It can be used to join tables that have columns with the same name and compatible data types.
Answer: BD
答案解析：
参考：http://blog.csdn.net/rlhua/article/details/12877591
A,最多可以连接三个表，错误，可以连接三个以上的表
B,限制自然连接的等值连接数，正确，因为当有多个列相匹配时，使用USING子句可仅与一列相匹配，NATURAL JOIN和USING语句是互相排斥的。
C,错误，USING只能用在等值连接，不能用在非等值连接。
D,列的名称相同和数据类型匹配，则可以连接表，正确。