trying to write a bit of logic as one query, can't seem to do it under 2
I'm trying to write a bit of logic as 1 query, but I can't seem to do
it under 2 queries.
i'm hoping someone can help
the basic premise is that i have an inventory management system , and
am trying to update the quantity available in the "shopping
cart" (which is different than the independently tracked quantity
requested ).
the logic is fairly simple:
cart items should show the quantity_requested as available if that
number is <= the number of items in stock, otherwise they should show
the max number of items available
the solution i ended up with, is to just update the cart_items with
the entire quantity_available per product, and then fix that in a
second pass.
i'm wondering if this can be *efficiently* done within a single update
statement. i couldn't figure out how to do this in a single update,
and not make multiple queries to find the actual qty_available
UPDATE
cart_item
SET
qty_requested_available = ( SELECT qty_available FROM stock where
stock.id = stock_id)
;
UPDATE
cart_item
SET
qty_requested_available =
CASE
WHEN
qty_requested_available > qty_requested THEN qty_requested
ELSE
qty_requested_available
END
;
Jonathan Vanasco <postgres@2xlp.com> writes:
I'm trying to write a bit of logic as 1 query, but I can't seem to do
it under 2 queries.
Uh, why can't you just push that CASE expression into the sub-select?
UPDATE
cart_item
SET
qty_requested_available =
( SELECT CASE
WHEN
qty_available > qty_requested THEN qty_requested
ELSE
qty_available
END
FROM stock where stock.id = stock_id )
;
You might have to qualify qty_requested here to make sure it comes from
cart_item, if there's a column of the same name in stock.
BTW, I'd suggest using GREATEST() instead of the CASE, but that's
just a minor improvement.
regards, tom lane
Tom Lane wrote:
Jonathan Vanasco <postgres@2xlp.com> writes:
I'm trying to write a bit of logic as 1 query, but I can't seem to do
it under 2 queries.Uh, why can't you just push that CASE expression into the sub-select?
<QUERY SNIPPED>
You might have to qualify qty_requested here to make sure it comes from
cart_item, if there's a column of the same name in stock.BTW, I'd suggest using GREATEST() instead of the CASE, but that's
just a minor improvement.
Like so?
UPDATE
cart_item
SET
qty_requested_available = least(cart_item.qty_requested,
stock.qty_available)
FROM
stock
WHERE
cart_item.stock_id = stock.stock_id AND
qty_requested_available <> least(cart_item.qty_requested,
stock.qty_available);
Also note the qualifier that prevents the query from updating every
cart_item row whether it needs it or not.
-Glen
it would be that, but with greatest
thank you. that's the exact query i was failing to write !
On Apr 21, 2010, at 8:51 PM, Glen Parker wrote:
Show quoted text
UPDATE
cart_item
SET
qty_requested_available = least(cart_item.qty_requested,
stock.qty_available)
FROM
stock
WHERE
cart_item.stock_id = stock.stock_id AND
qty_requested_available <> least(cart_item.qty_requested,
stock.qty_available);
Jonathan Vanasco wrote:
it would be that, but with greatest
Not if qty_requested_available needs to be <= qty_available...
-Glen