Modify a non-empty table field type Oracle

Execute the following statement "to modify the data type, the column you want to change must be empty"
ALTER TABLE name modify (Target Field VARCHAR2 (100));

Steps to resolve:

The first step is to add a temporary field to the table
ALTER TABLE name add temporary field the original type of the Target field;

The second step, the value of the target field is paid to the temporary field, and the Target field is empty
Update table Name set TEMP field = Target Field, Target Field =null;

Step three, modify the target type
ALTER TABLE name modify Target Field VARCHAR2 (100);

Fourth, the value of the temporary field is paid to the target field, and the temporary field is placed empty
Update table name set Target Field = Temp field, temporary field =null;

Last step, delete temporary fields
ALTER TABLE Name drop column temp field;