How to write such a query

Hi,
Consider following

[code]
CREATE TABLE X(id INTEGER PRIMARY KEY, field1 char(50), field2 int);
CREATE TABLE Y(id INTEGER PRIMARY KEY, field1 char, field2 double(10, 2));
SELECT X.field1, Y.field2 from X, Y WHERE X.id = Y.id;
[/code]

Assuming that the SELECT return 10 rows, I want to update X.field1
in row 5.

How do I write a WHERE clause in the

[code]
UPDATE X.field1 SET X.field1 = '<some_string>' WHERE....
[/code]

Thank you.