Oracle 12c --Invisible Columns 可隐藏列

3/7/2017来源:SQL技巧人气:486

Invisible Columns 可隐藏列 ( Oracle 12c release 1版本引入新功能 )         An invisible column is a user-specified column whose values are only visible when the column is explicitly specified by name. You can add an invisible column to a table without affecting existing applications, and make the column visible if necessary.         一个隐藏列的列是在指定列的值仅列时可见显式指定的名字。您可以添加一个看不见的列一个表在不影响现有应用程序,如果必要的,可以使列可见。         In general, invisible columns help migrate and evolve online applications. A use case might be an application that queries a three-column table with a SELECT * statement. Adding a fourth column to the table would break the application, which expects three columns of data. Adding a fourth invisible column makes the application function normally. A developer can then alter the application to handle a fourth column, and make the column visible when the application goes live.         一般来说,看不见列帮助迁移和发展在线应用程序。一个用例可能是一个应用程序,该应用程序可以查询一个三列的表 SELECT *声明。第四列添加到表会破坏应用程序,预计三列的数据。添加第四个无形的列使应用程序正常运转。开发人员可以改变应用程序来处理第四列,并使列可见当应用程序上线。         The following example creates a table PRoducts with an invisible column count, and then makes the invisible column visible:         下面的示例创建一个表 products一个隐藏列 ,然后使无形的列count可见:         CREATE TABLE products ( prod_id INT, count INT INVISIBLE );         ALTER TABLE products MODIFY ( count VISIBLE );     See Also:         Oracle Database Administrator’s Guide to learn how to manage invisible columns         Oracle Database SQL Language Reference for more information about invisible columns     参见:         Oracle数据库管理员指南学习如何管理无形的列         Oracle数据库SQL语言参考关于看不见的更多信息列