Query Optimisation required

Hi,

What I have right now is not directly a Postgres question, but more of a sql question for MS-Access and MS-SQL.
The application is a small Inventory Management Module in which there are 7 tables

Master Tables
1. ItemMaster containing the ItemID as the primary key
Transaction Tables - All of these tables are having a corresponding Transaction Master tables
2. PurchaseDetails , (e.g., child of PurchaseMaster)
3. SalesDetails
4. PurRetDetails (Purchase Returns)
5. SalesRetDetails (Sales Returns)
6 & 7. OtherIssues and OtherReceipts

ItemMaster PurchaseDetails SalesDetails PurReturns SalesRet
========== ================ ============ ========== ========
ItemID (PK) PurDetailsID (PK) SalesDetailsID (PK) PurRetID (PK) SalesRetID (PK)
ItemName ItemID (FK) ItemID (FK) ItemID (FK) ItemID (FK)
. Qty Qty Qty Qty
. . . . .
Stock . . . .

Similarly, there is otherissues and otherreceipts tables.

The thing is that the ItemID need not exist in all tables, so I need a Left Join. Though explained step-wise below, it is to be done by one query.
ItemMaster -> (Left Join) -> PurchaseDetails = ResultA (Add Qty to Stock)
ResultA -> (Left Join) -> SalesDetails = ResultB (Reduce Qty, etc)
ResultB -> (Left Join) -> PurReturns = ResultC
ResultC -> (Left Join) -> SalesRet = ResultD
..........

Currently, I have created independent views (queries), which left joins ItemMaster with each of the transaction tables and then one query which equi-joins each of the queries because of problems in Multiple Left-joins from one table in MS-Access (all RDBMSs give that prob ???)

The syntax I need should be so generic that if I am moving from MS-Access to MySQL or Postgres, I need not modify the code in my VB Application.
****I need this query for MS-Access, MS-SQL and Postgres. Because the application is supposed to be such that for upto 2-3 users, the application would be running on MS-Access or MSDE, while for heavier databases i.e., greater than 4 , the ideal database would be Postgres.

Thankx a lot
Kapil