Thanks to swampboogie for his help with this post http://www.sitepoint.com/forums/showthread.php?t=175861 but I now need to join a third table so I can filter companies by to the account handler (salesperson).

Heres a reminder of my two-table left join which work quite well

Code:
SELECT 
  companies.company_id, companies.name
FROM 
  companies
LEFT JOIN cost_centres ON companies.company_id = cost_centres.company_id
AND cost_centres.date >= '1086044400'
AND cost_centres.date <= '1086130799'
WHERE 
  cost_centres.company_id IS NULL
In a third table 'contact' there is a 'company_id' field that links 'contact' to 'companies' and a 'salesperson' field. I need to be able to select companies who's accounts are handled by a particular 'salesperson' if they have haven't accessed the site in a given period

Stew