if condition UPDATE else SET to zero

[code green]

I have two tables.
`tempTable` has two fields `part` and `qty`
`products` also has `part` and `qty` fields and a `last_updated` date stamp.
I wish to UPDATE `products`.`qty` with `tempTable.`qty` and date stamp it
BUT only if the `qty` fields differ.
Straightforward enough

UPDATE `products` JOIN `tempTable` USING(`part`)
SET `products`.`qty` = `tempTable`.`qty`,
`products`.last_updated` = NOW()
WHERE `products`.`qty` != `tempTable`.`qty`

But if the part exists in products but not in tempTable I wish to set `products`.`qty` to zero.

Can this be done in one query?
I am sure it can with some combination of IF ELSE / IF EXISTS but I am struggling (just back of holiday)
Two queries is fine but I would prefer one.
Any help appreciated

[pbmods]

Heya, Code Green.

Something like this, perhaps:

UPDATE
    (
            `products`
        JOIN
            `tempTable`
                USING
                    (`part`)
    )
    SET
        `products`.`qty` = `tempTable`.`qty`,
        `products`.last_updated` = NOW(),
        `products`.`qty` = IF
        (
            EXISTS
            (
                SELECT
                        *
                    FROM
                        `tempTable`
                    WHERE
                        `part` = `products`.`part`
            ),
            `products`.`qty`,
 
        )
    WHERE
        `products`.`qty` != `tempTable`.`qty`
 

Not sure if this will work, and at any rate, it's probably no faster than doing two queries anyway.

[code green]

Thanks pbmods. That looks quite impressive.
I'll give it a try but like you suspect, sub-queries are very slow.