What is the difference between column_name and qual_col_name?

qualified_col_name

specifies the fully qualified column path for XML tables. It basically stores an expression for that column. For regular columns, it will be column_name

.

If you run:

select owner, table_name, column_name, data_type, qualified_col_name
  from all_tab_cols
 where column_name <> qualified_col_name;

      

        
        
        
      

    

you will see the returned columns from XML tables in the XDB schema.

For example:

OWNER  TABLE_NAME       COLUMN_NAME   DATA_TYPE           QUALIFIED_COL_NAME
XDB    XDB$SIMPLE_TYPE  SYS_NC00074$  XDB$APPINFO_LIST_T  "XMLDATA"."RESTRICTION"."MIN_INCLUSIVE"."ANNOTATION"."APPINFO"

      

        
        
        
      

    

Update: For entity tables, it qualified_col_name

stores an expression of type casting and access to an attribute. For example:

create or replace type test1_obj as object(
   n1 number,
   n2 number,
   s1 varchar2(10),
   s2 varchar2(20)
)
not final;

create or replace type test2_obj under test1_obj(
  d1 date,
  d2 date
)
not final;

create table object_table of test1_obj;

select column_name, data_type, qualified_col_name 
  from user_tab_cols
where table_name = 'OBJECT_TABLE'
 order by internal_column_id;

      

        
        
        
      

    

For the last 2 hidden system columns, reserved for instances of type test2_obj, we can see:

COLUMN_NAME     DATA_TYPE   QUALIFIED_COL_NAME
SYS_NC00010$    DATE        TREAT(SYS_NC_ROWINFO$ AS "TEST"."TEST2_OBJ")."D1"
SYS_NC00011$    DATE        TREAT(SYS_NC_ROWINFO$ AS "TEST"."TEST2_OBJ")."D2"