Firebird 5.0

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select rdb$get_context('SYSTEM', 'ENGINE_VERSION') as version
     , rdb$character_set_name
from rdb$database;

VERSION
RDB$CHARACTER_SET_NAME

5.0.4
UTF8

-- partial indexes arrived in Firebird 5.0
create table reading (sensor int, v int);
create index ix_positive on reading (sensor) where v > 0;
insert into reading values (1, 5);
insert into reading values (2, -5);

1 row affected

1 row affected

select sensor, v from reading order by sensor;

SENSOR
V

1
5

2
-5

-- so did MERGE ... WHEN NOT MATCHED BY SOURCE, which can delete
create table target (id int, v int);
insert into target values (1, 1);
insert into target values (2, 2);
create table src (id int);
insert into src values (1);
merge into target t using src s on t.id = s.id
  when not matched by source then delete;
select * from target order by id;

1 row affected

1 row affected

1 row affected

1 row affected

ID
V

1
1