Обсуждение: UPDATE query based on a SELECT command.

Let's suppose that I have an UPDATE query which includes a SELECT as part of it as follows:

JORGE MALDONADO <jorgemal1960@gmail.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