Hi Scott,
Scott Haneda wrote:
Is it possible to use user variables in an update statement, I can find only
scant docs on it, but they do not pertain to what I am trying to do.
Given this select:
SELECT user_id, order_status, quantity_chosen, month_price, each_price,
sales_tax, sales_tax_rate,
@NEW_each_price:=(each_price + .06) as NEW_each_price,
@NEW_month_price:=(quantity_chosen * @NEW_each_price) as NEW_month_price,
@postage_rate:=(0.30) as postage_rate,
@cost_of_postage:=(quantity_chosen * @postage_rate) as postage,
@taxable_price:=(@NEW_month_price - @cost_of_postage) as taxable,
(@taxable_price * sales_tax_rate) as NEW_sales_tax
As you can see, I am using variables to make some calcs, sure, I can do it
long hand, but it gets long and confusing.
Given the above, I end up with some vars like NEW_month_price,
NEW_each_price etc, I would like to update ... SET price = NEW_each_price,
but of course, that does not work.
Suggestions?
It's possible, but I'm not sure it was ever intended. Still, I have found some very
useful applications for it. The trick is to put the assignment inside a function,
which will a) make sure the assignment happens b) present the assignment as an
expression that can go on the right-hand-side of a SET clause.
I explained in detail how it works here:
http://www.xaprb.com/blog/2006/12/15/advanced-mysql-user-variable-techniques/
But only read that if you want the background, which is a little tangential for your
purposes. Read this instead:
update tbl set
-- Set each_price to its present value, while setting @NEW_each_price
each_price = greatest(least(0, @NEW_each_price := each_price + .06)),
price = @NEW_each_price,
....
You can see some complex examples of that technique in this article:
http://www.oreillynet.com/pub/a/mysql/2007/03/01/optimize-mysql-rank-data.html?page=3
Following the examples in that article, you can do a lot more flexible things than I
just demonstrated.
--
Baron Schwartz
http://www.xaprb.com/
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