Dynamic SUM( ) in open SQL

* -----------------------------------------------------------------------------------
* Dynamic SUM( ) in openSQL
* -----------------------------------------------------------------------------------
* You want to sum the values of a table's column, but decide at runtime WHICH of
* the columns actually is to be summed up (e.g. by a decision of the user)
* In ABAP openSQL, it is possible to have a dynamic expression for the columns
* to be selected. This idiom, in principle, can be used to compute a sum dynamically
* But there are two things to have in mind:
* - the SQL syntax checker doesn't now that the dynamic colexp uses an aggregate function
*   so it complains that it expects an ENDSELECT, unless you specifiy "select single"
* - the string template for the column expression cannot be specified directly in the
*   select statement (again, the syntax checker is disturbed - this time by the round
*   brackets inside the literal part of the string template).
*   Therefore, the auxiliary variable lv_colexp is necessary for this solution.
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  data: lv_count type i.
  data(lv_sumfield) = 'NBRTAB'.

  data(lv_colexp) = |sum( { lv_sumfield } )|.
  select single (lv_colexp) from cccflow where uname = 'DDIC' into @lv_count.
  write / lv_count.