i have to update column .there is one to many relationship between two tables.and i am trying to do this ..

update line set passcode= "(select mdn from line l,line_esn le where l.line_id=le.line_id )"

passcode value is last 4 digit of mdn. mdn is 10 digit number.so how do i substring the mdn.likemdn.substring(6,10) .should i have to use function..

Use substr as follows

select substr('1234567890',7,4) from dual
OUPUT
7890

update line set passcode= (select substr(mdn,7,4) from line l,line_esn le where l.line_id=le.line_id )

Here you are using = .

If there is a chance to return more than one row then use distinct keyword in the subquery.

SELECT DISTINCT SUBSTR(MDN,7,4) from line l,line_esn le where l.line_id=le.line_id )