SQLPlus has a default number format is uses to display data on the screen "nicely".  You need to either 

o set the numformat to something you like
o use a TO_CHAR on the number with a format that applies in your case.

ops$tkyte@ORA817DEV.US.ORACLE.COM> select   TO_NUMBER( TO_CHAR(9999999.6765722378,'999999999D9999999999'))   from 
  2  dual;

TO_NUMBER(TO_CHAR(9999999.6765722378,'999999999D9999999999'))
-------------------------------------------------------------
                                                   9999999.68

ops$tkyte@ORA817DEV.US.ORACLE.COM> 
ops$tkyte@ORA817DEV.US.ORACLE.COM> set numformat 9999999999999.9999999999999
ops$tkyte@ORA817DEV.US.ORACLE.COM> /

TO_NUMBER(TO_CHAR(9999999.6765722378,'999999999D9999999999'))
-------------------------------------------------------------
                                        9999999.6765722378000

ops$tkyte@ORA817DEV.US.ORACLE.COM> 


See the difference?  I would recommand using a TO_CHAR with an explicit format when selecting the data out or using numformat. 

ops$tkyte@ORA920.LOCALHOST> select t.mycost, round(t.mycost,precision)
  2    from t1 t;

    MYCOST ROUND(T.MYCOST,PRECISION)
---------- -------------------------
  102.1234                    102.12
  102.1235                   102.124


cast does not work like that, cannot look work like that.


the TYPE of a column must be the same for all rows -- you would be trying to change the datatype row by row.  that'll never fly. 

that is a really strange request. but, we can do:

ops$tkyte@ORA9IR2> select x, to_number( to_char( x, '9.99EEEE' ) ) from t
  2  /
 
         X TO_NUMBER(TO_CHAR(X,'9.99EEEE'))
---------- --------------------------------
    105125                           105000
    105687                           106000
   .011223                            .0112
  .0012367                           .00124
 
 

ops$tkyte@ORA9IR2> select x, lpad(trunc(x),6) || rpad(decode(x-trunc(x),0,' ',x-trunc(x)),4) b
  2    from t;
 
         X B
---------- ----------
    123456 123456
    123.45    123.45
     1.235      1.235
       .55      0.55
  1254.423   1254.423
 


is one approach. 

sqlplus has a default number format for display purposes.

I assume you are using sqlplus.


SQL> set numformat 99999999999999.99

or whatever you like (heck, use to_char to be "sure" it works in everyones sqlplus if you are createing reports/scripts) 

what do you mean "additional" decimal points.

You would be showing a CONSISTENT set of decimal points (eg: always two of them"


In sqlplus, you  use 

SQL> set numformat <whatever)

or you use to_char.


ops$tkyte@ORA10GR2> select * from t;

         X
----------
-1.138E+11
     100.2
  30000.25
      3233

ops$tkyte@ORA10GR2>
ops$tkyte@ORA10GR2> select to_char( x, '999999999999999.99' ) from t;

TO_CHAR(X,'99999999
-------------------
   -113804083029.00
             100.20
           30000.25
            3233.00


or if you want

ops$tkyte@ORA10GR2> select case when trunc(x)=x
  2              then to_char( x, '999999999999999999' )
  3              else to_char( x, '999999999999999.99' )
  4              end x
  5     from t;

X
-------------------
      -113804083029
             100.20
           30000.25
               3233

 

0.00 is a string

0 is a number


if you insert the number zero, it is quite trivial to get the string 0.00


ops$tkyte@ORA10GR2> select to_char(x,'990.00') from t;

TO_CHAR
-------
   0.00
 123.45
 

ops$tkyte%ORA10GR2> select to_char( 1, 's999') from dual;

TO_C
----
  +1

ops$tkyte%ORA10GR2> select to_char( -1, 's999') from dual;

TO_C
----
  -1



you might want to check out the sql reference guide - lots of formats for you! 

ops$tkyte%ORA10GR2> create table t ( x varchar2(20) check (to_number(x) <> null));

Table created.

ops$tkyte%ORA10GR2> insert into t values ( '9.00000' );

1 row created.

ops$tkyte%ORA10GR2> insert into t values ( null );

1 row created.

ops$tkyte%ORA10GR2> insert into t values ( 'a' );
insert into t values ( 'a' )
*
ERROR at line 1:
ORA-01722: invalid number


ops$tkyte%ORA10GR2> select rownum, x from t;

    ROWNUM X
---------- --------------------
         1 9.00000
         2

ops$tkyte%ORA10GR2>

ops$tkyte%ORA11GR1> select c1||'|'||
  2         case when c2 < 1 then 0 end || to_char(c2) || '|' ||
  3         c3
  4    from test1
  5  /

C1||'|'||CASEWHENC2<1THEN0END||TO_CHAR(C2)||'|'||C3
-------------------------------------------------------------------------------
start|187027.760713112328881060040868595477561|end
start|0.5|end

ops$tkyte%ORA11GR2> select rownum, nvl2( x, to_char(x,'990.00')||'%', null )
  2    from (select '134.32' x from dual union all select null x from dual)
  3  /

    ROWNUM NVL2(X,T
---------- --------
         1  134.32%
         2

ops$tkyte%ORA11GR2> 

create table t (
  x number
);

insert into t values (720.95000000000005);
insert into t values (720.95);

select trim(
  to_char(x,'999G999G999G990D99999999999999','nls_numeric_characters = '',.''')
), trim(
  to_char(round(x, 2), '999G999G999G990D99999999999999','nls_numeric_characters = '',.''')
)
from t;

TRIM(TO_CHAR(X,'999G999G999G990 TRIM(TO_CHAR(ROUND(X,2),'999G99
------------------------------- -------------------------------
720,95000000000005              720,95000000000000             
720,95000000000000              720,95000000000000