My query is missing the alter number and default... How can I corrected.

Question

USER_A
create table TBL_A (       FIELD_A1 number not null,
FIELD_A2 varchar2(50),
FIELD_A3 date,
FIELD_A4 number(5,2) default 0,
FIELD_A5 varchar2(10) not null
USER_B
create table TBL_A (       FIELD_A1 number not null,
FIELD_A2 varchar2(20),
FIELD_A4 number(5,2) not null,
FIELD_A5 varchar2(10),
FIELD_A6 number(2) not null
I see the result like below after running below query only error is second line and rest is correct so how can I correct second line? :
alter table TBL_A modify FIELD_A5 VARCHAR2  (10)  not null
alter table TBL_A modify FIELD_A4 NUMBER 5, 2) null                    // want to see only this one like alter table TBL_A modify FIELD_A4 NUMBER (5,2) default 0,   rest is correct!
alter table TBL_A modify FIELD_A2 VARCHAR2  (50)  null
alter table TBL_A drop FIELD_A6
alter table TBL_A add FIELD_A3   DATE
MYQUERY:
with src as
  select src.table_name src_table_name, src.column_name src_col_name, src.data_type src_data_type, src.data_length src_data_len, src.data_precision src_data_precision, src.data_scale src_data_scale,
        src.nullable src_nullable,decode(T.Constraint_Type,'P', 'Primary Key','U','Unique','') as src_cons
    from  all_tab_columns src
    left join (select Cc.Column_Name,Uc.Constraint_Type
                 from user_cons_columns cc, user_constraints uc
                 where Cc.Constraint_Name = Uc.Constraint_Name
                   and Cc.Table_Name = Uc.Table_Name) t
      on T.Column_Name = Src.Column_Name
   where table_name = 'TBL_A' and owner='USER_A'
tgt as
  select tgt.table_name tgt_table_name, tgt.column_name tgt_col_name, tgt.data_type tgt_data_type, tgt.data_length tgt_data_len,
   tgt.data_precision tgt_data_precision, tgt.data_scale tgt_data_scale, tgt.nullable tgt_nullable,
   decode(T.Constraint_Type,'P', 'Primary Key','U','Unique','') as tgt_cons
    from all_tab_columns tgt
     left join (select Cc.Column_Name,Uc.Constraint_Type
                 from user_cons_columns cc, user_constraints uc
                 where Cc.Constraint_Name = Uc.Constraint_Name
                   and Cc.Table_Name = Uc.Table_Name) t
      on T.Column_Name = tgt.Column_Name
   where table_name = 'TBL_A' and owner='USER_B'
col_details as
  select src.src_table_name, nvl(tgt.tgt_table_name, first_value(tgt_table_name) over(order by tgt_table_name nulls last)) tgt_table_name,
         src.src_col_name, src.src_data_type, src.src_data_len, src.src_data_precision, src.src_data_scale, src.src_nullable,src_cons,
         tgt.tgt_col_name, tgt.tgt_data_type, tgt.tgt_data_len, tgt.tgt_data_precision, tgt.tgt_data_scale, tgt.tgt_nullable,tgt_cons
    from src
    full outer join tgt
      on (
          src.src_col_name = tgt.tgt_col_name
select *
  from (
        select case
                when tgt_data_type != src_data_type or tgt_data_len != src_data_len or tgt_data_precision != src_data_precision or tgt_data_scale != src_data_scale or tgt_nullable != src_nullable
                  then 'alter table ' || tgt_table_name || ' modify ' || tgt_col_name || ' ' || src_data_type || ' ' ||
                  case when src_data_type in ('DATE') then null
                       else
                            case
                              when src_data_type in ('VARCHAR', 'VARCHAR2')
                                then ' (' ||nvl(to_char(src_data_len), ' ') || ') '
                              else  decode(nvl(src_data_precision, -1), -1, null, nvl(to_char(src_data_precision), ' ') || ', ' || nvl(to_char(src_data_scale), ' ') || ')')
                            end
                  end
                  ||
                  case when src_nullable = 'Y' then ' null '
                    else ' not null ' end
                  || tgt_cons
                when tgt_col_name is null
                  then 'alter table ' || tgt_table_name || ' add ' || src_col_name || ' ' ||  ' ' || ' ' || src_data_type || ' ' ||
                  case when src_data_type in ('DATE') then null
                       else
                            case
                              when src_data_type in ('VARCHAR', 'VARCHAR2')
                                then '('|| nvl(to_char(src_data_len), ' ') || ') '
                              else  decode(nvl(src_data_precision, -1), -1, null, nvl(to_char(src_data_precision), ' ') || ', ' || nvl(to_char(src_data_scale), ' ') || ')')
                            end
                  end
                || tgt_cons
               when src_col_name is null
                 then 'alter table '|| tgt_table_name ||' drop '||tgt_col_name
               end alter_statement
          from col_details
where alter_statement is not null;

Answer 1

Stop reinventing the wheel. Oracle provides DBMS_METADATA_DIFF package:
SQL> desc u1.tbl_a
Name                                      Null?    Type
FIELD_A1                                  NOT NULL NUMBER
FIELD_A2                                           VARCHAR2(20)
FIELD_A4                                  NOT NULL NUMBER(5,2)
FIELD_A5                                           VARCHAR2(10)
FIELD_A6                                  NOT NULL NUMBER(2)
SQL> desc u2.tbl_a
Name                                      Null?    Type
FIELD_A1                                  NOT NULL NUMBER
FIELD_A2                                           VARCHAR2(50)
FIELD_A3                                           DATE
FIELD_A4                                           NUMBER(5,2)
FIELD_A5                                  NOT NULL VARCHAR2(10)
SQL> select dbms_metadata_diff.compare_alter('TABLE','TBL_A','TBL_A','U1','U2') from dual
  2  /
DBMS_METADATA_DIFF.COMPARE_ALTER('TABLE','TBL_A','TBL_A','U1','U2')
ALTER TABLE "U1"."TBL_A" ADD ("FIELD_A3" DATE)
  ALTER TABLE "U1"."TBL_A" DROP ("FIELD_A6")
  ALTER TABLE "U1"."TBL_A" MODIFY ("FIELD_A2" VARCHAR2(50))
  ALTER TABLE "U1"."TBL_A" MODIFY ("FIELD_A4" NUMBER(5,2) DEFAULT 0)
  ALTER TABLE "U1"."TBL_A" MODIFY ("FIELD_A4" NULL)
  ALTER TABLE "U1"."TBL_A" MODIFY ("FIELD_A5" NOT NULL ENABLE)
SQL>
SY.