主键
teamno int not null,
playerno int not null,
division char(6) not null,
primary key(teamno)
)
create table feng(
primary key
playerno int not null,
division char(6) not null,
)
复合主键
teamno int not null,
playerno int not null,
division char(6) not null,
primary key(teamno， playerno )
)
替代键（候选键）
primary key
playerno int not null,
division char(6) not null,
unique(playerno)
)
primary key
playerno int not null,
division char(6) not null,
unique(playerno，division )
)
外键（在innoDB中使用）
外键声明包括三个部分
1，那个列或列组合是外键
2，指定外键参照的表和列
3，参照动作[cascade（级联操作）,restrict（拒绝操作）,set null（设为空）,no action,set default]
如果没有指定参照动作默认是
on update restrict
on delete restrict
teamno int not null primary key ,
playerno int not null,
division char(6) not null,
foreign key（division）
references othertable （column）
on update restrict
unique(playerno)
)
check完整性约束
create table players
(playerno int not null,
sex char(1) not null,
check (sex in ('m','f'))
)
create table players
(playerno int not null,
birth_date date,
sex char(1) not null,
check (sex in ('m','f'))
joined smallint not null,
check (year(birth_date)<joined),
check (joined<1880),
)
create table players
(playerno int not null,
birth_date date,
sex char(1) not null,
check (sex in ( select sex from wholetab )
)
