SqlServer support for xml path

Src/Asp.Net/SqlSugar/ExpressionsToSql/DbMethods/DefaultDbMethod.cs

@@ -561,5 +561,9 @@ namespace SqlSugar
         {
             return null;
         }
+        public virtual string GetForXmlPath() 
+        {
+            return null;
+        }
     }
 }

Src/Asp.Net/SqlSugar/ExpressionsToSql/DbMethods/IDbMethods.cs

@@ -88,5 +88,6 @@ namespace SqlSugar
         string Desc(MethodCallExpressionModel model);
         string Stuff(MethodCallExpressionModel model);
         string GetDateString(string dateValue,string format);
+        string GetForXmlPath();
     }
 }

Src/Asp.Net/SqlSugar/ExpressionsToSql/Subquery/SubResolve.cs

@@ -19,6 +19,7 @@ namespace SqlSugar
         private ExpressionContext context = null;
         private string subKey = "$SubAs:";
         private bool hasWhere;
+        private bool isXmlPath = false;
         public SubResolve(MethodCallExpression expression, ExpressionContext context, Expression oppsiteExpression)
         {
             this.context = context;
@@ -124,6 +125,14 @@ namespace SqlSugar
             {
                 sql = string.Join(UtilConstants.Space, sqlItems);
             }
+            if (isXmlPath)
+            {
+                var xmlPath = context.DbMehtods.GetForXmlPath();
+                if (xmlPath.HasValue()) 
+                {
+                    sql = sql + xmlPath;
+                }
+            }
             return this.context.DbMehtods.Pack(sql);
         }
 
@@ -185,6 +194,10 @@ namespace SqlSugar
                  {
                      item = items.First(s => s is SubAndIF);
                  }
+                 else if (item is SubSelectStringJoin) 
+                 {
+                     isXmlPath = true;
+                 }
 
                  item.Context = this.context;
                  item.Expression = exp;

Src/Asp.Net/SqlSugar/Realization/SqlServer/SqlBuilder/SqlServerExpressionContext.cs

@@ -20,6 +20,14 @@ namespace SqlSugar
     }
     public partial class SqlServerMethod : DefaultDbMethod, IDbMethods
     {
+        public override string GetForXmlPath()
+        {
+            return "  FOR XML PATH('')),1,len(N','),'')  ";
+        }
+        public override string GetStringJoinSelector(string result, string separator)
+        {
+            return $"stuff((SELECT cast(N'{separator}' as nvarchar(max)) + cast({result} as nvarchar(max))";
+        }
         public override string DateValue(MethodCallExpressionModel model)
         {
             var parameter = model.Args[0];