Tennis balls and tennis rackets are complements. If a 3 percent change in the price of a tennis racket leads to a 9 percent change in the quantity of tennis balls demanded, the cross elasticity of demand equals
【简答题】有如下职员数据库表:职员(职员号(C,3),姓名(C,6),性别(C,2),职务(C,10))为职员表的"性别"字段设置有效性规则(只能取"男"或"女"),正确的SQL语句是A. ALTER TABLE 职员 ALTER 性别 CHECK 性别 $ "男女" B. ALTER TABLE 职员 ADD 性别 CHECK 性别 $ "男女" C. ALTER TABLE 职员 ALTER 性别 SE...