It gives exception when I specified the query

strQuery = "FROM Sample1 as S1 OUTER JOIN Sample2 as S2 on S1.Id=S2.Id"


Exception : ex = {"Error performing LoadByUniqueKey[SQL: SQL not available]"}
at NHibernate.Type.EntityType.LoadByUniqueKey(String entityName, String uniqueKeyPropertyName, Object key, ISessionImplementor session)
at NHibernate.Type.EntityType.ResolveIdentifier(Object value, ISessionImplementor session, Object owner)
at NHibernate.Engine.TwoPhaseLoad.InitializeEntity(Object entity, Boolean readOnly, ISessionImplementor session, PreLoadEvent preLoadEvent, PostLoadEvent postLoadEvent)
at NHibernate.Loader.Loader.InitializeEntitiesAndCollections(IList hydratedObjects, Object resultSetId, ISessionImplementor session, Boolean readOnly)
at NHibernate.Loader.Loader.DoQuery(ISessionImplementor session, QueryParameters queryParameters, Boolean returnProxies)
at NHibernate.Loader.Loader.DoQueryAndInitializeNonLazyCollections(ISessionImplementor session, QueryParameters queryParameters, Boolean returnProxies)
at NHibernate.Loader.Loader.DoList(ISessionImplementor session, QueryParameters queryParameters)
at NHibernate.Loader.Loader.ListIgnoreQueryCache(ISessionImplementor session, QueryParameters queryParameters)
at NHibernate.Loader.Loader.List(ISessionImplementor session, QueryParameters queryParameters, ISet`1 querySpaces, IType[] resultTypes)
at NHibernate.Hql.Ast.ANTLR.Loader.QueryLoader.List(ISessionImplementor session, QueryParameters queryParameters)
at NHibernate.Hql.Ast.ANTLR.QueryTranslatorImpl.List(ISessionImplementor session, QueryParameters queryParameters)
at NHibernate.Engine.Query.HQLQueryPlan.PerformList(QueryParameters queryParameters, ISessionImplementor session, IList results)
at NHibernate.Impl.SessionImpl.List(String query, QueryParameters queryParameters, IList results)
at NHibernate.Impl.SessionImpl.List(String query, QueryParameters parameters)
at NHibernate.Impl.QueryImpl.List()
at FirstDB.Program.Main(String[] args)