merge into SOME_TABLE as tbl
using ( select :PARAM_1 as KEY_FIELD,
               :PARAM_2 as FIELD_2,
               :PARAM_3 as FIELD_3
          from RDB$DATABASE ) as src
on ( tbl.KEY_FIELD = src.KEY_FIELD )
when not matched then ...

update or insert into SOME_TABLE (
    KEY_FIELD,
    FIELD_2,
    FIELD_3 )
values (
    :PARAM_1,
    :PARAM_2,
    :PARAM_3 )
matching ( KEY_FIELD )

create table T (
    ID   integer not null,
    VAL  varchar(16),
    FIX  timestamp
);

execute block
as
  declare variable CNT integer;
begin
  CNT = 0;
  while ( CNT < 1000000 ) do
  begin
    merge into T as tbl
    using ( select 777000 as ID,
                   'NEW FIELD VALUE!' as VAL
              from RDB$DATABASE ) as src
    on ( tbl.ID = src.ID )
    when not matched then
      insert ( VAL ) values ( src.VAL )
    when matched then
      update
         set VAL = src.VAL;
    CNT = :CNT + 1;
  end
end

execute block
as
  declare variable CNT integer;
begin
  CNT = 0;
  while ( CNT < 1000000 ) do
  begin
    update or insert into T (
        ID,
        VAL )
    values (
        777000,
        'NEW FIELD VALUE!' )
    matching ( ID );

    CNT = :CNT + 1;
  end
end