create table myTable (id INT, myData varchar(20));

insert into myTable VALUES
(1, 'Value 1'),
(2, 'Value 2'),
(3, 'Value 3');

insert into myTable (id) values (4);

update myTable SET myData=(select myData FROM myTable WHERE id=3) WHERE id=4;

update myTable
SET myData=(select * from (select myData FROM myTable WHERE id=3) x)
WHERE id=4;

update myTable left_table
inner join myTable right_table ON right_table.id = 3
set left_table.myData = right_table.myData
where left_table.id = 4;