数据库约束有关问题

数据库约束问题
问题补充：
现在有个表 
create table A(
Id int identity(100,1) primary key,
Name varchar(32) not null,
ParentId int
)

现在字段PARENTID想要加个约束 要求只能选择ID内已存在的值 问语句怎么写

[解决办法]
使用外键约束即可。

SQL code

mysql> create table A(    ->  Id int auto_increment primary key,    ->  Name varchar(32) not null,    ->  ParentId int,    ->  FOREIGN KEY (ParentId)  references a(id)    -> ) ENGINE=innodb;Query OK, 0 rows affected (0.07 sec)mysql> insert into a values (null,'AAAA',null);Query OK, 1 row affected (0.07 sec)mysql> insert into a values (null,'BBBB',3);ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`csdn`.`a`, CONSTRAINT `a_ibfk_1` FOREIGN KEY (`ParentId`) REFERENCES `a`(`Id`))mysql> insert into a values (null,'BBBB',1);Query OK, 1 row affected (0.11 sec)mysql>