Postgres 18

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create table franck ( val int unique );

CREATE TABLE

insert into franck values (-1),(1) ;

INSERT 0 2

select val from franck;

val

-1

1

SELECT 2

update franck set val=-val;

ERROR:  duplicate key value violates unique constraint "franck_val_key"
DETAIL:  Key (val)=(1) already exists.

select val from franck;

val

-1

1

SELECT 2