MySQL中建立外键问题

        哪位高手能给发一些有关MySQL中建立外键约束的详细介绍资料，本人在table中建立外键约束时总是提示同一个错误“ERROR 1005 (HY000): Can't create table 'xxx' (errno: 121)”，不知该如何解决，因此在建table时，不敢加外键约束，在网上收了很久都没找到有关的详解，很是苦恼。
          还请各位高人给予指教！

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你的MYSQL版本是多少？
用的什么存储引擎？

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SQL code:
mysql> create table t1 (
-> id int primary key,
-> col int
-> )engine=innodb;
Query OK, 0 rows affected (0.09 sec)

mysql> create table t2 (
-> id int primary key,
-> tid int,
-> FOREIGN KEY (tid) REFERENCES t1(id)
-> )engine=innodb;
Query OK, 0 rows affected (0.09 sec)

mysql> insert into t2 values (2,1);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint f
ails (`csdn`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`tid`) REFERENCES `t1` (`
id`))
mysql> insert into t2 values (1,null);
Query OK, 1 row affected (0.05 sec)

mysql> insert into t1 values (1,91);
Query OK, 1 row affected (0.06 sec)

mysql> insert into t2 values (2,1);
Query OK, 1 row affected (0.03 sec)

mysql> select * from t2;
+----+------+
| id | tid |
+----+------+
| 1 | NULL |
| 2 | 1 |
+----+------+
2 rows in set (0.00 sec)