select 
lodo.oper_date,
lodo.operation,
sum(lodo.qty),
case (select count(*) from LIST_OPER_WORKERS xxx where xxx.oper_id = lodo.id)
    when 1 then low.worker_id
    else '---'
end
from LIST_OPER_WORKERS low
inner join LIST_ORDER_DET_OPER lodo on lodo.id = low.oper_id
group by
lodo.oper_date,
lodo.operation

can't format message 13:896 -- message system code -4.
Dynamic SQL Error.
SQL error code = -104.
Invalid expression in the select list (not contained in either an aggregate function or the GROUP BY clause).