Why? Well, first we'll look at what an Ambiguous Outer Join is, and then see how to correct it.
Microsoft Access has three types of joins: the Inner Join, the Right Join and the Left Join. Both the Right and Left joins are known as Outer Joins. An Inner Join shows only those records that exist in both tables. However, an Outer Join (both Right and Left) shows all of the records from one table (the Base Table) and just the matching records from the other (Secondary Table).
When Access processes a multiple table query, it needs to determine the order in which to join the tables. Should it join Table1 to Table2 first and then join Table3? Or should it do it in some other order? This is part of the Rushmore technology of the Jet engine. It tries to determine the most efficient way to process the query.
In the case of standard Inner Joins, no matter what order the tables are joined the result set will be the same. However, this is not the case with Outer Joins. There are times, when using an Outer Join, that the result of the query will be different depending on the order the tables are joined. When this happens, you have an Ambiguous Outer Join.
So how do you know when an Outer Join will result in an error? The easiest way to understand it is in terms of what you see in the Query Builder grid.
A table which participates in an Outer join as a Secondary Table (that is, the arrow is pointing *towards* it) cannot participate in either an Inner Join, or as a Secondary Table in another Outer Join. Figure 1 shows two types of queries that will result in an Ambiguous Outer Join error.
Figure 1: Two illegal Outer Join Queries
However, the table participating in the Outer Join as a Secondary Table can participate in another Outer Join if it is the Base table of the other Outer Join (that is, the arrow points *away* from it). Figure 2 shows a query that will not result in an Ambiguous Outer Join error.
Figure 2: A legal Outer Join Query
So what do you do if you need to create a query like case 1 or 2? You have to split the query into a stacked query, that is, two queries, the second of which uses the first. This is exactly what the Ambiguous Outer Join error message suggests.
Create a query joining the first two tables with an Outer Join and save it as a named query (i.e. Query1). Then, in a second query, join the first query to the third table. Figure 3 shows how to build a stacked query.
Figure 3: Shows how to create a query, which will not have an Ambiguous Outer Join by splitting it into two queries.
So the Ambiguous Outer Join error is not really all that confusing. It simply means that the database wants you to decide which join it should create first. In Access, you do this by spitting the query into a stacked query.