A quick note to help me remember how to do this.
The problem: you want to select the smallest value from a set of values.
Let's say you have a table of products that are in a logical group and you want to select the lowest priced product from that group, however some products actually have a 0.00 price (for whatever reason). You don't want to actually show 0.00 as the lowest price for this group of products, you want to show the lowest price that happens to be greater than zero.
MySQL has a neat way to do this. Simply go:
SELECT tableref.group_id, MIN(NULLIF(tableref.column, 0)) as min_price FROM tableref GROUP BY tableref.group_id;
The magic is in the NULLIF function, which will return null if tableref.column is equal to 0. Returning null removes that value from inclusion by MIN, having the effect of forcing the column value to be greater than zero.