SQL级联更新和级联删除

alter table 表名
add constraint 约束名
foreign key(字段名) references 主表名(字段名)
on delete cascade

语法：
Foreign Key
(column[,...n])
references referenced_table_name[(ref_column[,...n])]
[on delete cascade]
[on update cascade]
注释：
column:列名
referenced_table_name:外键参考的主键表名称
ref_name:外键要参考的表的主键列
on delete:删除级联
on update:更新级联

表的外键约束 SELECT 外键表ID=b.fkeyid ,外键表名称=object_name(b.fkeyid) ,外键列ID=b.fkey ,外键列名=(SELECT name FROM syscolumns WHERE colid=b.fkey AND id=b.fkeyid) ,主键表ID=b.rkeyid ,主键表名=object_name(b.rkeyid) ,主键列ID=b.rkey ,主键列名=(SELECT name FROM syscolumns WHERE colid=b.rkey AND id=b.rkeyid) FROM sysobjects a join sysforeignkeys b on a.id=b.constid join sysobjects c on a.parent_obj=c.id where a.xtype='f' AND c.xtype='U'                   查询一个表的全部外键: SELECT 主键列ID=b.rkey ,主键列名=(SELECT name FROM syscolumns WHERE colid=b.rkey AND id=b.rkeyid) ,外键表ID=b.fkeyid ,外键表名称=object_name(b.fkeyid) ,外键列ID=b.fkey ,外键列名=(SELECT name FROM syscolumns WHERE colid=b.fkey AND id=b.fkeyid) FROM sysobjects a join sysforeignkeys b on a.id=b.constid join sysobjects c on a.parent_obj=c.id where a.xtype='f' AND c.xtype='U' and object_name(b.rkeyid)='要查询的表名'