LOOP AT... GROUP BY with dynamic group key - grouping

Question

I am trying to loop by grouping data with dynamic group parameter.
We can use dynamic queries on WHERE conditions on loops but I do not know whether it is possible to use a dynamic string in group condition.
Here is the sample where user decides by which field to group and then put additional logic based on the decision:
DATA query TYPE string.
IF i_condition_type = 'ERNAM'.
query = |ERNAM = MARA-ERNAM|.
ELSE.
query = |ERSDA = MARA-ERSDA|.
ENDIF.
LOOP AT lt_mara INTO DATA(mara) GROUP BY ( (query) ) "syntax error
ASSIGNING FIELD-SYMBOL(<group>).
LOOP AT GROUP <group> ASSIGNING FIELD-SYMBOL(<line_data>).
"//do something
ENDLOOP.
ENDLOOP.
Is there any way to do this? I am also open to other ideas besides grouping because if I can't dynamically group, I will copy so many lines and change only the group key.

Answer 1

As pointed out in the comments, LOOP AT ... GROUP BY doesn't support dynamic group-by clauses from strings.
In this simple example you could create your grouping key dynamically at runtime by creating it with an inline expression like COND or SWITCH:
LOOP AT lt_mara INTO DATA(mara) GROUP BY
SWITCH string(
i_condition_type
WHEN 'ERNAM' THEN mara-ernam
WHEN 'ERSDA' THEN mara-ersda
ELSE ''
)
But your key-building logic might be too complex to express with an expression (or at least an expression which is still readable by a human being). In that case there is something else you can do: group on values returned by a method:
LOOP AT lt_mara INTO DATA(mara)
GROUP BY my_grouping_method( line = mara
condition = i_condition_type )
The implementation of that method can then include any logic you need to form the grouping key at runtime:
METHOD my_grouping_method.
IF condition = 'ERNAM'.
result = line-ernam.
ELSE.
result = line-ersda.
ENDIF.
ENDMETHOD.
The grouping method can also be a method of a different object. So you could represent your grouping condition as an own class. That would allow you to write code like this:
DATA(lo_group_condition) = NEW zcl_mara_group_condition( 'ERNAM' ).
LOOP AT lt_mara INTO DATA(mara)
GROUP BY lo_group_condition->get_key_from( mara )