JORGE MALDONADO <jorgemal1960(at)gmail(dot)com> writes:
> Let's suppose that I have an UPDATE query which includes a SELECT as part
> of it as follows:
> UPDATE table1
> SET field1 = (SELECT fieldx FROM table2 WHERE .........)
> What happens if the result of the SELECT command does not return any row?
A scalar subquery that returns no rows has a result of NULL. This is
not specific to whether it's used in UPDATE. See
http://www.postgresql.org/docs/9.1/static/sql-expressions.html#SQL-SYNTAX-SCALAR-SUBQUERIES
regards, tom lane