C#:

public static void Query(IOrganizationService service)
        {
            //实体名称
            string entityName = "new_sp";
            RetrieveEntityRequest request = new RetrieveEntityRequest();
            request.LogicalName = entityName;
            RetrieveEntityResponse response = (RetrieveEntityResponse)service.Execute(request);
            //主字段名称
            string name = response.EntityMetadata.PrimaryNameAttribute;
        }

sql:

--查询表的主字段
select top 1 name,ColumnNumber from MetadataSchema.Attribute where entityid in
(select entityid from MetadataSchema.Entity where name = ‘new_sp‘)
and length = 200
order by ColumnNumber