关于create database link的有关问题。快点进来，多谢

关于create database link的问题。急，快点进来，谢谢
我是这样写的
  create database link db1_link
  connect to user identified by pwd
  using '(DESCRIPTION =
  (ADDRESS_LIST =
  (ADDRESS = (PROTOCOL = TCP)(HOST = liuip)(PORT = 1521))
  )
  (CONNECT_DATA =
  (SERVICE_NAME = database_name)
  )
  )';


其实很简单，但是问题是执行报错。说 identified by pwd pwd有问题 
我在考虑要不要加单引号'pwd' 加了也不行，不加也报错。为什么为什么？？急。

------解决方案--------------------

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我是这样写的
create database link db1_link
connect to user identified by pwd
using '(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = liuip)(PORT = 1521))
)
(CONNECT_DAT……