Get Products from database

I'm trying to get all products from the database with a single query. I get stuck at the price part:

VirtueMart has a table called #__vm_product and another one called #__vm_product_price.

If a product has a parent product, it means that product inherits everything from the parent unless it's set different in the child.

The tables look like this:

/* #__vm_product PARTIAL */
int - product_id
int - product_parent_id
varchar - product_name

/* #__vm_product_price PARTIAL */
int - product_id
decimal - product_price
int - mdate

I made the next query which gets all products with their price:

SELECT
p.product_id AS id,
product_name AS name,
product_price AS price,
p.product_parent_id AS parent,
MAX(pp.mdate) AS last_updated

FROM jos_vm_product p
LEFT JOIN jos_vm_product_price pp ON p.product_id = pp.product_id

GROUP BY p.product_id
ORDER BY p.product_id

The problem with th开发者_Python百科is query is that it doesn't check if their is a price specified. So if it's a child product and it has no price, it should show the price of it's parent.

Could someone help me out with this?

Note: If anyone knows an easier way to get all products (with price) from the VirtueMart database, please don't mind to tell me :)

EDIT: Price is never null. If child is supposed to inherit from it's parent it just doesn't have a price row in jos_vm_product_price

Updated:

select
    p.product_id AS id, 
    p.product_name AS name, 
    coalesce(pp.product_price, pp2.product_price) AS price, 
    p.product_parent_id AS parent, 
    coalesce(pp.mdate, pp2.mdate) AS last_updated 
from jos_vm_product p 
left outer join jos_vm_product p2 on p.product_parent_id = p2.product_id
left outer join (
    select product_id, max(mdate) as maxmdate
    from jos_vm_product_price
    group by product_id
) ppm on p.product_id = ppm.product_id
left outer join jos_vm_product_price pp on ppm.product_id = pp.product_id and ppm.maxmdate = pp.mdate 
left outer join (
    select product_id, max(mdate) as maxmdate
    from jos_vm_product_price
    group by product_id
) ppm2 on p2.product_id = ppm2.product_id
left outer join jos_vm_product_price pp2 on ppm2.product_id = pp2.product_id and ppm2.maxmdate = pp2.mdate 

You can use a CASE statement to check the product price for the child and if it's null, use the parent price.

Also, you need another join to get the parent.

    SELECT
        p.product_id AS id,
        product_name AS name,
        CASE
            WHEN pp.product_price IS NULL then pp2.product_price
            ELSE pp.product_price
        END AS price,
        p.product_parent_id AS parent,
        MAX(pp.mdate) AS last_updated

    FROM jos_vm_product p
    LEFT OUTER JOIN jos_vm_product_price pp
           ON p.product_id = pp.product_id
       , jos_vm_product parent
       , jos_vm_product_price pp2
   WHERE (p.product_parent_id = parent.product_id OR p.product_parent_id IS NULL)
     AND (parent.product_id = pp2.product_id OR p.product_parent_id IS NULL)
    GROUP BY
        p.product_id,
        product_name,
        CASE
           WHEN pp.product_price IS NULL then pp2.product_price
           ELSE pp.product_price
        END,
        p.product_parent_id