Thread: Complex Update Queries with Fromlist

In Microsoft SQL Server, I can write an UPDATE query as follows:

update orders set RequiredDate =
(case when c.City IN ('Seattle','Portland') then o.OrderDate + 2  else o.OrderDate + 1 end)
from orders o
join customers c on
o.Customerid = c.Customerid
where c.region in ('WA','OR')

This query finds 47 rows matching the WHERE clause and updates the RequiredDate in the Orders table based on data in the orders table and the customer table for these 47 rows.

It appears that I can do the same thing in Postgres with the following syntax:

update orders set RequiredDate =
(case when c.city in ('Seattle','Portland') then date(o.OrderDate) + 1 
   else date(o.OrderDate) + 2 end)
from orders o
join customers c on
o.Customerid = c.Customerid
where c.region in ('WA','OR')
and orders.orderid = o.orderid

The only difference being that I need to add the join at the end to join the orders table in the update statement with the "orders o" table in the fromlist.

First, does this look correct?  It appears to work the way I want.  Second, it would be really nice if there was better documentation of the UPDATE statement in Postgres, including examples of this type.

Thanks. 

Mark Dexter
Dexter + Chaney
9700 Lake City Way NE, Seattle, WA  98115-2347
Direct Phone: 206.777.6819  Fax: 206-367-9613
General Phone: 800-875-1400 
Email: mdexter@dexterchaney.com

Mark Dexter wrote:
>
> update orders set RequiredDate =
> (case when c.city in ('Seattle','Portland') then date(o.OrderDate) + 1
>    else date(o.OrderDate) + 2 end)
> from orders o
> join customers c on
> o.Customerid = c.Customerid
> where c.region in ('WA','OR')
> and orders.orderid = o.orderid
>
> The only difference being that I need to add the join at the end to join
> the orders table in the update statement with the "orders o" table in
> the fromlist.

That's because of the explicit join you're using. The "orders o" in the
FROM clause is different from the "orders" table in the UPDATE clause.

I'd probably use something like:

UPDATE orders
   SET RequiredDate = ...
FROM
   customers c
WHERE
   orders.Customerid = c.Customerid
   AND c.region in (...)


> First, does this look correct?  It appears to work the way I want.
> Second, it would be really nice if there was better documentation of the
> UPDATE statement in Postgres, including examples of this type.

Patches to the documentation are always gratefully received. The latest
version of the documentation is available on the main website (follow
the developers link). Contributions to the docs mailing-list in plain
text are generally fine.

--
   Richard Huxton
   Archonet Ltd

Thank you.  That works for the simple example I had.

However, I have an additional question about this.  What if I need to do
a LEFT OUTER JOIN in the Fromlist.  For example, this query works but it
requires the orders table to appear twice.

UPDATE orders set requireddate =
  (case when c.city in ('Seattle','Portland') then date(o.OrderDate) + 1

    else date(o.OrderDate) + 2 end)
FROM orders o
LEFT OUTER JOIN customers c on
  o.customerid = c.customerid
  where c.region in ('WA','OR')
 and orders.orderid = o.orderid

Is there some way to do an outer join in the Fromlist back to the main
table being updated without having it appear twice?  Thanks.