Best way to implement value check in database using PHP - php

Question

I'm very new to PHP. I'm building a companion webservice for an iPhone application I'm creating.
I have the following DB table that i've created which is filled with triggers.
The basic concept is that for a given currency when the price hits the trigger price I'm going to send a push notification.
Before I can generate the push notification I need an elegant way to validate each entry in this table against the current market price.
On initial thought I was thinking to select each trigger price and each currency and compare against the current market price I have in another variable.
Could anyone tell me the most elegant way to achieve what I'm trying to do? (compare the whole table against market price for various currencies)
Also if you have any php snippets that would be great,
Thanks,
John

Answer 1

Well if the app has a user logged in then you only need to check for the rows for that user, which isn't that bad.
However I would implement the database differently, have a separate currencies table like this:
id SERIAL
currency_name VARCHAR(10)
market_price FLOAT
Then change your main table so that target_price is a FLOAT as well (you could use int if you store the number of cents -- or equivalent -- instead of a dollar value, so 103 instead of 1.03). Finally change currency to be an int that references the other table
currency_id INTEGER
Then you probably want to add a foreign key constraint to the table:
FOREIGN KEY (currency_id)
REFERENCES currencies(id)
Now you can update the currencies table as the market_price changes then use SQL to get a list of the rows that need triggered:
SELECT u.id
FROM users u, currencies c
WHERE u.currency_id = c.id
AND u.trigger_price <= c.market_price
;
That's probably the best way, in my opinion.
If you really need to do it in PHP then I'd create an associative array of the market prices where prices['currency_name'] = market price for that currency. And currency name matches the string in the table. Then you're just going to have to go through the whole table and check each match, assuming you've already fetched the table into $result and checked for errors:
while ($row = $result->fetch_assoc()) {
if (isset($prices[$row->currency) // Sanity check on the prices array
&& ($row->trigger_price <= $prices[$row->currency]))
{
// Matching row, do something with it ...
}
}
If you trust the database to only contain stuff in the currency array you can skip the sanity check. If you only want to check certain currencies you could only select rows that match currencies you have updates for.
I'm also assuming your condition is the trigger price being lower than the current price, you may need to do different logic depending on exactly what you want to do.