SQL - update table problem...

Hi,

first i'm sorry if this message appears twice. first time didn't get through 
because i wasn't registered.

i'm trying to do update on multiple tables but don't know how.
is something like this posible with postgresql?

update table1
join table2 on (table1.id=table2.t1)
join table3 on (table2.id=table3.t2)
set table1.name='test', table2.sum=table1.x+table2.y, 
table3.cell='123456789'
where table1.id=6

i know that this syntax is not supported with postgres but i tried to 
rewrite the code using this synopsis:

UPDATE [ ONLY ] table SET column = { expression | DEFAULT } [, ...]  [ FROM fromlist ]  [ WHERE condition ]

i failed again.

thanks for help

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