PHP MYSQL加入并输出无与伦比的结果

Question

I've two tables : One called "Collected_items" and the Other "Wrong_Collected"..As shown below :

**collected_items**
item_no qty
x1       10
x2       20
BB       5
Z1       20
x13      13


**wrong_collected**
item_no qty
x1      10
x2      20
x13     13

As you can see, there are item no.'s occurred on both tables, but all i want is to output the one's which are not found in wrong_collected table.

$sql = mysql_query("SELECT collected_items.item_no,wrong_collected.item_no FROM collected_items, wrong_collected WHERE collected_items.item_no!=wrong_collected.item_no");
while($data = mysql_fetch_array($sql)){
   echo $data["item_no"];
}

The output shows all item no.'s except BB and Z1 ..Althout if you remove the ! from collected_items.item_no=wrong_collected.item_no the output will show the matching item_no's as follows :

X1 X2 X13

Sorry, I'm not really familiar with PHP. I've learned JOIN, but i never saw an example on how to output the unmatched results. Please Help !

Answer 1

change this:

SELECT collected_items.item_no,wrong_collected.item_no FROM collected_items, wrong_collected WHERE collected_items.item_no!=wrong_collected.item_no

to:

SELECT ci.item_no FROM collected_items ci 
left join wrong_collected wc on ci.item_no=wc.item_no 
WHERE wc.item_no is null;