Can we change datatype without deleting records ?

Question

Hi all ;
Without deleting records in a table  , can we  change datatype in oracle 10g and 11g ?
SQL> desc emp
Name                                      Null?    Type
NO                                                 NUMBER
NAME                                            VARCHAR2(10)
CITY                                               VARCHAR2(15)
DEP_ID                                          NUMBER
SQL> select * from emp;
        NO NAME       CITY                DEP_ID
        1   madhir   calcutta                10
SQL> alter table emp modify(dep_id  varchar2(5));
alter table emp modify(dep_id varchar2(5))
ERROR at line 1:
ORA-01439: column to be modified must be empty to change datatype
SQL>  select * from v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0      Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

Answer 1

See 'Modifying an Existing Column Definition' in the DBA Guide
http://docs.oracle.com/cd/B28359_01/server.111/b28310/tables006.htm#i1006645
Use the ALTER TABLE...MODIFY statement to modify an existing column definition. You can modify column datatype, default value, column constraint, column expression (for virtual columns) and column encryption.
You can increase the length of an existing column, or decrease it, if all existing data satisfies the new length. You can change a column from byte semantics to CHAR semantics or vice versa. You must set the initialization parameter BLANK_TRIMMING=TRUE to decrease the length of a non-empty CHAR column.
If you are modifying a table to increase the length of a column of datatype CHAR, realize that this can be a time consuming operation and can require substantial additional storage, especially if the table contains many rows. This is because the CHAR value in each row must be blank-padded to satisfy the new column length.
And the ALTER TABLE statement in the SQL Language doc
http://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_3001.htm#SQLRF01001
modify_col_properties
Use this clause to modify the properties of the column. Any of the optional parts of the column definition (datatype, default value, or constraint) that you omit from this clause remain unchanged.
datatype You can change the datatype of any column if all rows of the column contain nulls. However, if you change the datatype of a column in a materialized view container table, then Oracle Database invalidates the corresponding materialized view.
You can omit the datatype only if the statement also designates the column as part of the foreign key of a referential integrity constraint. The database automatically assigns the column the same datatype as the corresponding column of the referenced key of the referential integrity constraint.
You can always increase the size of a character or raw column or the precision of a numeric column, whether or not all the rows contain nulls. You can reduce the size of a datatype of a column as long as the change does not require data to be modified.The database scans existing data and returns an error if data exists that exceeds the new length limit.