Correct way to write this query? I'm trying to write a query that basically is searching by price, though sometimes the items may have a discount on them and this price isn't stored in the DB, only the discount percent and dates the discount runs between are, so we have to work out the price of the item on the fly to see if it should be included.

Additionally, if the item is an auction we ignore the sale price and just look at the start_price and max_bid to work out the current price (max bid is only set if the item has at least 1 bid).

We know it is NOT an auction if start_price == buyout_price, so know we have to figure out if the item is on sale or not and if so calculate the sale price to see if to include it in their requested price range.

I have this code..

SELECT
    whatever
FROM
    auctions AS a
LEFT JOIN
    ..............
WHERE
    ..............
AND
    IF(a.start_price == a.buyout_price AND a.discount_start_date &lt; UNIX_TIMESTAMP() AND a.discount_end_date &gt; UNIX_TIMESTAMP() AND a.discount &gt; 0, (a.buyout_price - ((a.buyout_price / 100) * a.discount)), IF(a.max_bid &gt; a.start_price, a.max_bid, a.start_price)) &gt;= '500' AND IF(a.start_price == a.buyout_price AND UNIX_TIMESTAMP() &gt; a.discount_start_date AND UNIX_TIMESTAMP() &lt; a.discount_end_date AND a.discount &gt; 0, (a.buyout_price - ((a.buyout_price / 100) * a.discount)), IF(a.max_bid &gt; a.start_price, a.max_bid, a.start_price)) &lt;= '700'
AND
    ..............;


As you may be able to make out I'm trying to get all results with prices between 500 - 700.

I'm also getting a error which appears to have something to do with the UNIX_TIMESTAMP(), though not sure why!?

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '== a.buyout_price AND  a.discount_start_date &lt; UNIX_TIMESTAMP() AND a.discount_e' at line 1
Write yourself a view that produces, in a single column, the value of price that you're interested in.  It might be a UNION of a few different criteria.  When you're done you'll have a view of all items with two columns: the id and the price.  

Join that view to whatever else you need (perhaps including the original price), and voila.  

Tip: skip IF and just use AND and OR, per the SQL standard.  And eschew LEFT JOIN unless you know -- and know why -- you can't use a proper inner join.  If the database is pervasively unreliable in that regard, it will lead to more pain than all the outer joins in the world can alleviate.