Update MySql

Src/Asp.Net/MySqlTest/Demos/1_Query.cs

@@ -332,6 +332,7 @@ namespace OrmTest.Demo
         {
             var db = GetInstance();
             var t1 = db.Queryable<Student>().Where(it => SqlFunc.ToLower(it.Name) == SqlFunc.ToLower("JACK")).ToList();
+            var t2 = db.Queryable<Student>().Where(it => SqlFunc.IsNull(it.Name, "nullvalue") == "nullvalue").ToList();
             //SELECT [Id],[SchoolId],[Name],[CreateTime] FROM [Student]  WHERE ((LOWER([Name])) = (LOWER(@MethodConst0)) )
 
             /***More Functions***/

Src/Asp.Net/SqlSugar/Realization/MySql/SqlBuilder/MySqlExpressionContext.cs

@@ -123,5 +123,11 @@ namespace SqlSugar
         {
             return " concat("+string.Join(",", strings).Replace("+", "") + ") ";
         }
+        public override string IsNull(MethodCallExpressionModel model)
+        {
+            var parameter = model.Args[0];
+            var parameter1 = model.Args[1];
+            return string.Format("IFNULL({0},{1})", parameter.MemberName, parameter1.MemberName);
+        }
     }
 }