coding help for bonus code redeeming and preventing reusing same code

[APuppyDog]

hey gurus, i am a newbie php coder.. i am learning by example.

 

what i am trying to do is write a piece of code which will alter 3 tables (user, bonus_credit, bonus_credit_usage)

 

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the table structure that will be used is as follows:

 

user.bonus_credit

user.ID

 

bonus_credit.bonusCode

bonus_credit.qty

bonus_credit.value

 

bonus_credit_usage.bonusCode

bonus_credit_usage.usedBy

 

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so lets say, in bonus_credit i have the following

bonusCode = 'facebook' (this is the code they have to type to redeem the bonus

qty = '10' ( number of times the bonusCode can be redeemed, but same person can't redeem it more than once)

value = '5' (this is the  amount of bonus_credit for each qty)

 

Now, I need to write a code that check to see if the code has been redeemed in the bonus_credit_usage table and if the user.ID exists in this table as bonus_code_usage.usedBy, then give an error that its already been used and if it hasn't been used, then subtract 1 from qty, add ID to usedBy and then add the value to the bonus_credit

 

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i have started the steps just to create a simple textbox and entering a numeric value to bonus_credit, and that works.. but now i have to use JOIN and IF and ELSE.. which is a little too advanced for me.. so i'd appreciate a guide as i write the code.

 

	if(isset($_REQUEST['btnBonus']))
{	
	$bonus_credit = addslashes($_REQUEST['bonusCode']);

	$query = "update user set bonus_credit=bonus_credit+'".$bonus_credit."' where id='".$_SESSION['SESS_USERID']."'";
	echo "<script>window.location='myreferrals.php?msgs=2';</script>";
	mysql_query($query) or die(mysql_error());
}