Pervasive Sql 10 Join one table, onto another, onto another

Question

I have a table with products.
When I get information from that table, I would also like to get the ETA of that article. To do so, I am planning to get the latest purchase Order Row, that is on this article, and then get the expected delivery of this purchase.

This is three different tables and I would like it to be like another column on the query, so I can get the value from the column like I would if it was on the same table.

Is my idea possible? If there is no purchase order on this article I would like the value to be null.

Products

Int ProductId
Int Price

Sample data

ProductId  Price
-----------------
1          100
2          300

PORows

Int RowId
Int ProductId
Int POId

Sample data

RowId  ProductId  POId
-----------------------
1      1          1

PO

Int POId
DateTime ETA

Sample data

POId  ETA
-----------------------
1     2010-10-25 10:05

So the result I would want is:

ProductId   Price  ETA (null if no rows exist)
------------------------------------------------
1           100    2010-10-25 10:05
2           300    NULL

Answer 1

Use:

   SELECT p.productid,
          p.price,
          x.max_eta
     FROM PRODUCTS p
LEFT JOIN POROWS r ON r.productid = p.productid
LEFT JOIN (SELECT po.id,
                  MAX(po.eta) AS max_eta
             FROM PO po 
         GROUP BY po.id) x ON x.poid = r.poid

Pervasive is the only database I'm aware of that won't allow you to omit the INNER and OUTER keywords. v10 might've relaxed that, but I know it's the case for v8 and 2000.

Answer 2

I don't know Pervasive but in SQL standard you can make the select for the latest PO an aliased subquery

select Products.id, products.name, ProductETAS.ETA
from Products
left join

( 
  select POLINES.productid, min(PO.ETA) as ETA from PO inner join POLINES
   on PO.id = POLINES.POid and POLINES.productid = ?
   where PO.ETA >= today 
   group by POLINES.productid
 ) as ProductETAS

 on Products.productid = ProductETAS.productid