Let me give you a scenario and tell me if it applies to you. If not can you give me more specific info to assist you...
Example of 2 table; Employees and Sales.
Employees lists all of the companies employees.
Sales lists all the sales of each employee that has a sale.
The link between the two tables is on Employee ID.
Not all employees have sales.
If you inner join the tables (and query employees table after joining) you will now only be able to see employees from the employee table that have a corresponding sales record (e.g. none of the support employees or managers that do not do sales will appear).
If you left join employees table to the sales table you will get all of the employees from employees table.
However if you now add a select statement to your report of Sales.amount > 100.00 it will now render your left join useless because it is only going to select records where there is data in sales.
If you change your select statement to "isnull(sales.amount) or sales.amount > 100.00" now your left join works again to include all records from employees that do not match to sales at all and all the ones that match with sales amounts >100.
Hope this helps.
Edited by DBlank - 01 Jun 2009 at 1:25pm