其实如果仔细猜一下也应该能猜到：
因为CDB 是由root container,seed pdb container,0个或者多个pdb container组成的。
查询官档 v$pdbs视图：
CON_ID
The ID of the container to which the data pertains. Possible values include:
?0: This value is used for rows containing data that pertain to the entire CDB. This value is also used for rows in non-CDBs.
?1: This value is used for rows containing data that pertain to only the root
?n: Where n is the applicable container ID for the rows containing data
所以CON_ID =1 是rootcontainer，
原文：http://blog.51cto.com/2012ivan/2113977