mysql join getting a value or a null into a column

Question

I am very new to SQL, so I am having a couple problems figuring things out. The database is for an online store, and the structure is something like this:

users: UserID, UserName, etc.

pricelists: PricelistID, UserID, ProductItemID, Price

productitems: ProductItemID, ProductID, ItemID

products: ProductID, ManufacturerID, WebPageText, etc.

Each product can have one or more items (eg if the product page is selling T-shirts, there may be 5 different T-shirt variations for sale). That's pretty normal. What's not normal is that the pricing for each item is determined by what's in the user's pricelist, because each user is assigned particular pricing. Not all users have a price assigned for every item.

Here is the question: The requirement from management is that the user should be able to see every product, even if that product is not in the user's pricelist. If the user has no pricelist entry for a productitem, the page should display, "Contact us for pricing." I have not been able to figure out how to do this with one query. Every join that I attempted involving the pricelist table threw out products for which the user didn't have a price assigned. I am given only the ProductID and UserID. I have put my code below, which is a mix of mysql and PHP. It works, but it is clunky, especially seeing as I have to redo the second query over and over. Please tell me how I really ought to be doing it.

$query_product = sprintf("SELECT * , (items.Stock - (SELECT Coalesce(Sum(DetailQuantity),0) 
FROM orderdetails 
INNER JOIN orders ON OrderID = DetailOrderID 
INNER JOIN productitems ON orderdetails.ProductItemID = productitems.ProductItemID
INNER JOIN products ON productitems.ProductID = products.ProductID
INNER JOIN items ON productitems.ItemID = items.ItemID
WHERE OrderDate > ProductUpdateDate))*ProductLive 
AS NumLeft
 FROM productitems
 INNER JOIN products ON products.ProductID = productitems.ProductID
JOIN items ON productitems.ItemID = items.ItemID
WHERE products.ProductID = %s",GetSQLValueString($ProductID, "int"));
$product = mysql_query($query_products, $x) or die(mysql_error());
$row_product = mysql_fetch_assoc($product);

//after narrowing down to one productitem, either through user selections or while looping through $row_product:

$query_price = sprintf("SELECT Price FROM pricelists 
WHERE UserID = %s AND ProductItemID = %s", GetSQLValueString($UserID, "int"), GetSQLValueString($row_product['ProductItemID'], 'int'));
$price = mysql_query($query_price, $x) or die(mysql_error());
$row_price = mysql_fetch_assoc($price);
$row_product['Price'] = $row_price['Price'];

Later in the code, I check whether $row_products['Price'] is empty or not to determine what is displayed on the page.

EDIT: I thought this would be obvious, but ... I have to limit pricelists by WHERE UserID = %s. That means I can't just tack on a LEFT JOIN pricelists ON pricelists.ProductItemID = productitems.ProductItemID , which was actually the first thing I tried, and which does not work. Limiting the outer join in the WHERE statement turns it into an inner join.

Answer 1

Add the pricelists table as an LEFT join to your main query:

SELECT field1, field2, ..., pl.*
FROM orderdetails 
INNER JOIN orders ON OrderID = DetailOrderID 
...
LEFT JOIN pricelists pl PN pl.ProductItemID = productitems.ProductItemID
..

In your code check if the price in the pricelists table is null, if so, show the text.

Answer 2

You should look into using an outer join for this. Heres a great description of the difference between an inner and outer join -> inner join vs outer join