updates delete query to work with MySql

src/Umbraco.Core/Persistence/SqlSyntax/SqlSyntaxProviderExtensions.cs

@@ -22,12 +22,23 @@
         /// <remarks>
         /// See: http://issues.umbraco.org/issue/U4-3876
         /// </remarks>
-        public static Sql GetDeleteSubquery(this ISqlSyntaxProvider sqlProvider, string tableName, string columnName, Sql subQuery)
+        public static Sql GetDeleteSubquery(this ISqlSyntaxProvider sqlProvider, string tableName, string columnName, Sql subQuery, WhereInType whereInType = WhereInType.In)
         {
-            return new Sql(string.Format(@"DELETE FROM {0} WHERE {1} IN (SELECT {1} FROM ({2}) x)",
+
+            return 
+                new Sql(string.Format(
+                    whereInType == WhereInType.In
+                        ? @"DELETE FROM {0} WHERE {1} IN (SELECT {1} FROM ({2}) x)"
+                        : @"DELETE FROM {0} WHERE {1} NOT IN (SELECT {1} FROM ({2}) x)",
                 sqlProvider.GetQuotedTableName(tableName),
                 sqlProvider.GetQuotedColumnName(columnName),
                 subQuery.SQL), subQuery.Arguments);
         }
     }
+
+    internal enum WhereInType
+    {
+        In,
+        NotIn
+    }
 }