【单选题】关系模式department(dcode,name),employee(ecode,name,sex),修改employee表结构,为此表添加一个dept,此字段为外键(参照department表的主键、级联修改),正确的SQL语句是( )。
A.
alter table employee add dept varchar(4) foreign key references departments(dcode) on update cascade;
B.
alter table employee add dept varchar(4) foreign key;
C.
update table employee add dept varchar(4) references departments(dcode) on update cascade;
D.
alter table employee modify dept varchar(4) foreign key references departments(dcode) on update cascade;