drop table t3;
create table t3(
    id int primary key,
    c1 varchar(16) default null,
    c2 varchar(16) GENERATED ALWAYS AS (concat(c1, c1)) VIRTUAL NOT NULL,
    KEY idx (c2)
);

insert into t3(id, c1) values(1, "aaaa");
insert ignore into t3 set id = 1, c1 = "bbbb" on duplicate key update id = 2, c1 = null;
insert into t3 set id = 2, c1 = "cccc" on duplicate key update c1 = "dddd";