PSQL Constraint based on column value

Question

Is it possible to have a Constraint but only when one column is set to a particular value. For example take this pseudo-code example of a President which checks to make sure there is never more than 1 President at any time (note, this is NOT valid psql syntax)

CREATE TABLE president (
    id BIGSERIAL PRIMARY KEY,
    current BOOLEAN NOT NULL,
    CONSTRAINT there_can_be_only_one CHECK(COUNT(current=true)<=1)
);
    

Answer 1

You can use the so called partial index to enforce this specific constraint. In SQL Server they are called filtered indexes.

CREATE UNIQUE INDEX IX ON president (current)
WHERE current = true;

This index should prevent having more than one row in a table with current value set to true , because it is defined as unique.

Answer 2

Unfortunately NO as far as I know and anyway it already tells us,

ERROR: aggregate functions are not allowed in check constraints .

But we can use BEFORE trigger to check that the data you are trying to insert should meets the criteria COUNT(current=true)<=1

CREATE TABLE president (
    id BIGSERIAL PRIMARY KEY,
    current BOOLEAN NOT NULL
);
---------------------------------------------------------------------
CREATE FUNCTION check_current_flag() 
RETURNS trigger 
AS $current_president$
DECLARE
  current_flag_count integer;
BEGIN
  SELECT COUNT(*) FILTER (WHERE current = true )
    INTO current_flag_count 
    FROM president;
 
 IF    new.current = true
   and current_flag_count >= 1 THEN
   RAISE EXCEPTION 'There can be only one current president';
   -- RETURN NULL;
 ELSE
  RETURN NEW;
 END IF;
END;
$current_president$ LANGUAGE plpgsql;
---------------------------------------------------------------------
CREATE TRIGGER current_president BEFORE INSERT OR UPDATE ON president
    FOR EACH ROW EXECUTE PROCEDURE check_current_flag();

Db<>Fiddle for reference

Note: You can either throw exception in case of preconditions doesn't match ore simply returning NULL will skip the insert and do nothing. as official document says also here