The following bug has been logged online:

Bug reference: 2849
Logged by: Gurjeet Singh
Email address: singh.gurjeet (AT) gmail (DOT) com
PostgreSQL version: 8.2.0
Operating system: Windows
Description: Erroronous UPDATE syntax in documentation
Details:

The documentation at

http://www.postgresql.org/docs/8.2/i....html#AEN38727

illustrates the examples using wrong syntax of UPDATE command. The FROM
clause is missing; if the FROM clause and the appropriate table is added to
the query, it works fine.

Sample session:

postgres=# create table t1( a int, b int );
CREATE TABLE
postgres=# create table t2( a int, b int );
CREATE TABLE
postgres=# update t1 set b = t2.b where t1.a = t2.a;
ERROR: missing FROM-clause entry for table "t2"
LINE 1: update t1 set b = t2.b where t1.a = t2.a;
^
postgres=# update t1 set b = t2.b from t2 where t1.a = t2.a;
UPDATE 0
postgres=#

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