Add demo

Src/Asp.NetCore2/MySqlTest/Demo/Demo1_Queryable.cs

@@ -42,6 +42,11 @@ namespace OrmTest
             var getByWhere2 = db.Queryable<Order>().Where(it => it.Id == DateTime.Now.Year).ToList();
             var getByFuns = db.Queryable<Order>().Where(it => SqlFunc.IsNullOrEmpty(it.Name)).ToList();
             var getByFuns2 = db.Queryable<Order>().GroupBy(it => it.Name).Select(it => SqlFunc.AggregateDistinctCount(it.Price)).ToList();
+            var test47 = db.Queryable<Order>().Select(it => new
+            {
+                names = SqlFunc.Subqueryable<Order>().Where(z => z.Id == it.Id).SelectStringJoin(z => z.Name, ",")
+            })
+            .ToList();
             Console.WriteLine("#### Examples End ####");
         }