下面选项中,用于将子查询外层查询条件设置为内层查询返回的集合内的关键字是( )
A. IN
B. ON
C. WHERE
D. NOT IN
下面选项中,用来统计记录条数的函数是( )
A. avg()
B. length()
C. count()
D. total()
假定用户表user中存在一个字段age,现要 "查询年龄为18或20的用户" ,下面SQL语句中,正确的是( )
A. select * from user where age = 18 or age = 20;
B. select * from user where age = 18 && age= 20;
C. select * from user where age = 18 and age = 20;
D. select * from user where age = (18,20);
若主表的主键取值为1与2,那么从表相关联的外键取值是( )
A. 1
B. 2
C. 1或2
D. 任意值