Here is a good step-by-step tutorial that I followed
WCFDataService returns XML from Oracle database and is queryable through URLs

Sample Queries:
http://localhost:21759/WebSite28/WcfDataService.svc/
Query EMPLOYEES table
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES
Query for EMPLOYEE_ID = 100
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES(100M)
Query for the FIRST_NAME of EMPLOYEE_ID = 100
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES(100M)/FIRST_NAME
Count number of rows
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES/$count
Query first two EMPLOYEES results
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES?$top=2
Query for EMPLOYEES with LAST_NAME = KING
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES?$filter=LAST_NAME eq 'King'
Query for EMPLOYEES paid more than 10000 per pay period
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES?$filter=SALARY gt 10000
WCFDataService returns XML from Oracle database and is queryable through URLs

Sample Queries:
http://localhost:21759/WebSite28/WcfDataService.svc/
Query EMPLOYEES table
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES
Query for EMPLOYEE_ID = 100
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES(100M)
Query for the FIRST_NAME of EMPLOYEE_ID = 100
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES(100M)/FIRST_NAME
Count number of rows
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES/$count
Query first two EMPLOYEES results
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES?$top=2
Query for EMPLOYEES with LAST_NAME = KING
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES?$filter=LAST_NAME eq 'King'
Query for EMPLOYEES paid more than 10000 per pay period
http://localhost:21759/WebSite28/WcfDataService.svc/EMPLOYEES?$filter=SALARY gt 10000