change variables within select statement

Dear all, is there a possibility to change declared variables in monetdb within a select statement. For example in MySQL the variables can be declared with the @-symbol, and such variables easily could be changed in a case-when condition. Here a little example of what I want to do (in MySQL syntax): SET @id = ''; SET @pos = 1; drop table if exists test_table; create table test_table as select *, if (@id<> id, @pos:=1, @pos:=@pos+1) as position_number, @id:=id, from table_with_ids; Here the rows with identical ids should be counted, and if the id changes the position_number should start again as zero. If somebody has a hint for me, to execute such a query in monetdb, I would be very glad. Greetings, Christian