create table people(  Code Varchar(50) primary key,  Name Varchar (30) not null,  Time date,  Age int,  Nation varchar(50) references Nation2(Code) ); create table Nation2( Code varchar(50)  primary key, Name varchar(50) not null)create table Race(  Code varchar(50)  primary key,#注意不能设为自增长因为自增长必须为整形，否则报错  Name varchar (50)) 添加数据： insert into Nation2 values('001','汉族'); insert into Nation2 values ('002','回族'); insert into Nation2 values( '003','苗族');   insert into Nation2 values('004','壮族'); insert into Nation2 (Code) values('005');  insert into Nation2 (Name)values('蒙古族'); insert into Nation2 (Code) values('006');#多次试验发现Code带主键的可以多次单个添加code但name 只能第一次添加一个，再添加就报错。或者删了这个在添加也报错  delete from Nation2 where  Code=003;#删除 单个数据  delete from Race #删除整个数据    select*from people#查询people所有数据  select*from Nation2 where Code=4 #根据条件查询 drop table Nation2 #删除整张表  update Nation2 set Name='维吾尔族'where code=003;#修改某个数据 update people set Name='李四',Time='2000-06-03',Age='16',Nation='回族' where Code=001;# 修改时一定注意最后一个不能加逗号