显示数据库表间的外键关系

select foreign_key=b.name,
       foreign_key_table=c.name,
       foreign_key_columns
         =isnull((select name from syscolumns
                  where id=c.id and colid=a.fkey1),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey2),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey3),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey4),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey5),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey6),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey7),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey8),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey9),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey10),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey11),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey12),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey13),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey14),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey15),'')
         +isnull((select ','+name from syscolumns
                  where id=c.id and colid=a.fkey16),''),
       primary_key_table=d.name,
       primary_key_columns
         =isnull((select name from syscolumns
                  where id=d.id and colid=a.rkey1),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey2),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey3),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey4),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey5),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey6),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey7),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey8),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey9),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey10),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey11),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey12),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey13),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey14),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey15),'')
         +isnull((select ','+name from syscolumns
                  where id=d.id and colid=a.rkey16),''),
       cascade_update=case when (b.status/8192)%2=1 then 'Y' else '' end,
       cascade_delete=case when (b.status/4096)%2=1 then 'Y' else '' end
  from sysreferences a,sysobjects b,sysobjects c,sysobjects d
  where b.parent_obj in(select id from sysobjects
                        where xtype='U' and name<>N'dtproperties')
    and a.constid=b.id
    and a.fkeyid=c.id
    and a.rkeyid=d.id
  order by foreign_key_table,foreign_key_columns
### 如何在数据库中删除约束 在管理过程中,有时需要移除约束以调整数据结构或解决特定问题。以下是关于如何通过SQL语句删除约束的具体方法。 #### 删除约束的SQL语法 要在MySQL或其他支持标准SQL的数据库管理系统中删除约束,需使用`ALTER TABLE`语句并指定具体的名称。此操作的前提是已知的确切名称。如果不确定名称,则可以通过查询系统元数据获取相关信息[^1]。 ```sql -- 基本语法:删除约束 ALTER TABLE_name DROP FOREIGN KEY 名; ``` 例如,假设存在一张名为`orders`的子,并且该有一个`fk_customer_id`指向另一张的主: ```sql -- 示例:删除名为 fk_customer_id 的 ALTER TABLE orders DROP FOREIGN KEY fk_customer_id; ``` #### 查询名称的方法 当忘记或者不清楚具体名称时,可通过查询`INFORMATION_SCHEMA.KEY_COLUMN_USAGE`视图定位目标的信息。以下是一个通用查询示例: ```sql SELECT CONSTRAINT_NAME, TABLE_NAME, COLUMN_NAME, REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE WHERE TABLE_NAME = '子_name' AND REFERENCED_TABLE_NAME = '主_name'; ``` 上述查询返回的结果将显示当前数据库的相关细节,包括其名称、所属列以及关联的目标和列。 #### 注意事项 - **数据完整性保护**:即使决定移除某个,在执行前仍应仔细评估可能带来的影响,因为这可能会破坏原有的参照完整性机制。 - **权限需求**:某些情况下,仅具有适当权限的用户才能修改或删除现有的定义。 #### 违反约束处理实例 对于已经存在的违反约束情况(比如父级记录被误删),除了修正逻辑错误之,还可以考虑清理受影响的数据集。例如,下面这条命令会清除学生成绩(`SC`)里所有成绩字段为空(NULL)的条目[^3]: ```sql DELETE FROM SC WHERE Grade IS NULL; ``` 尽管这不是直接针对的操作,但它展示了维护良好关系型模型的重要性——即及时发现并修复不一致之处。 ---
评论
添加红包

请填写红包祝福语或标题

红包个数最小为10个

红包金额最低5元

当前余额3.43前往充值 >
需支付:10.00
成就一亿技术人!
领取后你会自动成为博主和红包主的粉丝 规则
hope_wisdom
发出的红包
实付
使用余额支付
点击重新获取
扫码支付
钱包余额 0

抵扣说明:

1.余额是钱包充值的虚拟货币,按照1:1的比例进行支付金额的抵扣。
2.余额无法直接购买下载,可以购买VIP、付费专栏及课程。

余额充值