Compare two columns for difference mark the difference in mysql

I need to compare two fields and mark the similar fields in third table.

For Example:

Tables

customer: phone id, phonenumber followstatus

received_Customer: rc_id, rc_phonenumber.

I need to mark the fdnumber with f or n/a in follow up which is in received and customer table.

update customer select b.phonenumber from received_customer n join customer b on b.phonenumber = n.phonenumber SET followUpStatus= 'y' ;

I run query in php.