What is the difference between and inner join and an outer join?
An inner join will give you the records from your two tables that they have in common. So if you are "inner joining" customer ID, you will only get results if both tables share the same customer ID as a record. An outer join (more than likely a left outer join or just left join) will give you all records from your first table and only the records from your second table that are shared. So if the customer ID is in the first table but not in the second table, the results will still show the customer ID. This can be used if you know you do not want to eliminate any records in your first table because they may not be shared in your second table.
What is the VLOOKUP function and how do I use it?
VLOOKUP is a simple way to search a separate data source (usually another sheet within the current spreadsheet, or a new spreadsheet altogether) for a matching record in order to retrieve additional bits of information (located in the same row) about the record that you currently do not have in your main spreadsheet. Here's an example: John Doe has a social security number in your main spreadsheet but there's no date of birth for John Doe. You need to populate his date of birth in a particular field in this spreadsheet. You have a second spreadsheet with social security numbers and date of births, but no names. Since you already know the social security number exists in both spreadsheets, you can retrieve the date of birth for John Doe using the VLOOKUP function. It will "search" for John Doe's social security number in the second spreadsheet and can return any information in that matched row - in this case his date of birth.
How do I know which fields from each table I have selected in my query are the fields needed to establish links or joins, so that my query returns the results I expect?
The relationship between the tables is only as good as the unique identifiers in each table which are the primary keys. Think of it like this - At least two of the tables you're joining should have something to uniquely identify the records (for customer records, a customer ID is an example). A customer ID may or may not have a business, a phone number, or a contact in one or more tables, but those pieces of information will apply to a unique customer ID.