You can also use the FROM in SELECT, INSERTS, DELETE statements, because it is just another way to join tables.

So the (corrected) example code

update

a1

set

col = a2.col

from

table1 as a1,

table2 as a2

where

a1.col = a2.col

will execute the same operation as

update

a1

set

col = a2.col

from

table1 as a1

inner join table2 as a2

on a1.col = a2.col

When running operations with a big amount of rows, the second should have the better performance.

[EDIT]

This is because statement no.1 will first select all rows on both tables and then matches them with the where-clause. Statement no.2 will first match the tables with the join-clause and then select the matching rows. The performance will increase when the clause uses an index (i.e. primary key).

[/EDIT]