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
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
-> 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 f
ails (`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>