Postgres 14

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create table if not exists demo (
    id       serial primary key,
    a        int not null,
    b        int not null,
    current  boolean not null,
    constraint one_per unique (a, current)
);

INSERT INTO demo (a, b, current) VALUES (1, 101, true);
1 rows affected

--       ON CONFLICT ON CONSTRAINT one_per DO UPDATE SET current = false WHERE demo.id = EXCLUDED.id ;
CREATE OR REPLACE FUNCTION before_insert ()
RETURNS trigger LANGUAGE plpgsql AS
$$
BEGIN
  UPDATE demo
     SET current = false
   WHERE a = NEW.a ;
  RETURN NEW ;
END ;
$$ ;

CREATE TRIGGER before_insert BEFORE INSERT ON demo
FOR EACH ROW EXECUTE FUNCTION before_insert () ;
INSERT INTO demo (a, b, current)
       VALUES (1, 102, true)
1 rows affected

select * from demo


id
a
b
current


1
1
101
f

2
1
102
t