oracle11g - SQL Error: ORA-12899: Overflow inserting null in 30 character column -

i running simple insert select , code

insert temp select a.emp_name,        null address,        a.emp_id,        .......        temp1 a,temp2 b a.batch_id=b.batch_id 

now column address varchar2(30) though inserting null . got overflow .

sql error: ora-12899: value large column "temp"."address" (actual: 35, maximum: 30)

really puzzled how can happen.can provide tips? using oracle 11g

you putting wrong values columns, since haven't specified them in insert clause. it's better list columns explicitly , not rely on order appear in data dictionary:

insert temp (emp_name, address, empi_id, ...) select a.emp_name,    null,    a.emp_id,    ....... temp1 join temp2 b on a.batch_id=b.batch_id 

i've changed explicit join syntax, though isn't relevant question...