修改student(code,name,sex)表结构,为sex字段添加一个约束,确保sex的值只能取’男’或者’女’,正确的SQL语句是( )。
A.
check( sex in ('男','女'))
B.
constraint chk_student_sex check(sex in ('男','女'))
C.
alter table student add constraint chk_sex check( sex in ('男','女'));
D.
alter table student add chk_sex check( sex in ('男','女'));