SELECT * FROM products p LEFT JOIN criteria_values cv ON p.key=cv.key LEFT
JOIN criteria c ON cv.key=c.key WHERE c.value IS NOT NULL

Hard to answer without more detail, but I am guessing the answer will be
something like the above. Your question makes it hard to understand whether
c or cv is joined to p. So swap em around if I misunderstood. 

iPhone 4. It rocks!

On Nov 21, 2010, at 1:37 AM, Simcha Younger <> wrote:

> On Sat, 20 Nov 2010 13:54:29 -0700
> "Ben Miller" <> wrote:
>> Hi,
>> I'm building a website for a client in which I need to compare their 
>> products, side-by-side, but only include criteria for which all 
>> selected products have a value for that criteria.
>> In my database (MySQL), I have a tables named "products","criteria" 
>> and "criteria_values"
>> If I have something like
>> $selected_product = array("1"=>"Product 1","2"=>"Product 2"...)  //  
>> All products selected for comparison by the user
>> I need to get only rows from "criteria" where there is a row in 
>> "criteria_values" matching "criteria.criteria_id" for each 
>> $selected_product
>> - in other words, if any of the $selected_product does not have a row 
>> in "criteria_values" that matches "criteria.criteria_id", that 
>> criteria would not be returned.  I hope that makes sense.
> It would be a lot easier to think about this if you could provide the
table structure or create table statements.
> If I understood correctly, you have products which reference a criteria ID
which has no matching value. If this is the problem you have a to first take
care of the integrity of your data, as this should never happen. 

To help clarify - the 3 tables look something like the following (tableName
=> column,column,column...):

Products => product_id,product_name,product_description...  (key =
Criteria => criteria_id,criteria_title,criteria_text,...  (key =
Criteria_values => product_id,criteria_id,criteria_value,... (key =
product_id & criteria_id)

The user selects up to X product_id's to compare, stored in

I then need to get each criteria_title and criteria_text from
table(criteria) where there is a matching criteria_id in
table(criteria_values) for each/all $selected_products, also returning the
criteria_value for each $selected_products, ultimately ending up with an
array or object that looks something like:

(Assuming the user selected Product A (product_id=1), Product B
(product_id=2) and Product C (product_id=3)

criteria => Array  (
        [$criteria_id] => Array (
                [title] => query_row[criteria_title]
                [text] => query_row[criteria_text]
                [values] => Array (
                        [1] => Product A's value for this criteria
                        [2] => Product B's value for this criteria
                        [3] => Product C's value for this criteria
        [$criteria_id] => Array (

Again, displaying only/all criteria where there is a matching value for
each/all $selected_products

Thanks again,

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