Do SOA have to have web service for implementation?

SOA is a design principle which describes a certain way of communication and organization of discrete pieces of software components called services. It is leveraged usually in re-usable web services in the middle tier of the application with outstanding results. But it's core ideas can be utilized in any application where discrete components are present.

One aspect of SOA is "Decoupling". Two systems can be decoupled using an ESB. The interfaces could be as simple as flat files or can be decoupled by using Message Queues (for Asynchronous transactions).

In a simple way, SOA is about exposing business services (Like Invoices or IP address managment) as services that can be invoked wether by SOAP or EMS or even RPC as long it is decoupled from its system.

SOA is an application integration architecture wherein functional parts of applications are considered as seevices. If your looking into inhouse applications then you dont have to expose your service as webservice.