MSQL:Como substituir a entrada somente se a nova for maior? senão crie uma nova entrada

Eu usaria insert .. on duplicate key update ... declaração. Algo assim:

insert into highscore set
    name = :name,
    score = :new_score
on duplicate key update
    score = greatest(score, :new_score)

name coluna deve ser indexada como unique .

Roteiro de teste:

create table player (
    name varchar(32) primary key,
    score int not null default 0
);

-- create new players
insert into player set name = 'foo', score = 100
    on duplicate key update score = greatest(score, 100);
insert into player set name = 'bar', score = 100
    on duplicate key update score = greatest(score, 100);
insert into player set name = 'baz', score = 100
    on duplicate key update score = greatest(score, 100);

-- update score of existing player
insert into player set name = 'bar', score = 200
    on duplicate key update score = greatest(score, 200);

Saída de select * from player :

+------+-------+
| name | score |
+------+-------+
| bar  |   200 |
| baz  |   100 |
| foo  |   100 |
+------+-------+