Insert into a MySQL table or update if exists

Question

I want to add a row to a database table, but if a row exists with the same unique key I want to update the row.

For example:

Answer 1

When using SQLite:

REPLACE into table (id, name, age) values(1, "A", 19)

Provided that id is the primary key. Or else it just inserts another row. See INSERT (SQLite).