mysql数据库学习——4，完整性约束
主键
create table feng(teamno int not null,playerno int not null,pision char(6) not null,primary key(teamno))create table feng(teamno int not null primary key ,playerno int not null,pision char(6) not null,)
复合主键
create table feng(teamno int not null,playerno int not null,pision char(6) not null,primary key(teamno，playerno))
替代键（候选键）
create table feng(teamno int not null primary key ,playerno int not null,pision char(6) not null,unique(playerno))create table feng(teamno int not null primary key ,playerno int not null,pision char(6) not null,unique(playerno，pision))
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 restricton delete restrictcreate table feng(teamno int not null primary key ,playerno int not null,pision char(6) not null,foreign key（pision）references othertable （column）on update restrictunique(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)
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