The tables that are referenced in an ON clause must be part of the join that the ON clause modifies. For example, the following is invalid:

FROM ( A KEY JOIN B ) JOIN ( C JOIN D ON A.x = C.x )

The problem is that the join condition A.x = C.x references table A, which is not part of the join it modifies (in this case, C JOIN D).

However, as of the ANSI/ISO standard SQL99 and Adaptive Server Anywhere 7.0, there is an exception to this rule: if you use commas between table expressions, an ON condition of a join can reference a table that precedes it syntactically in the FROM clause. Therefore, the following is valid:

FROM (A KEY JOIN B) , (C JOIN D ON A.x = C.x)

See also: Key joins.

For more information about commas, see Commas.

 Example

SELECT Employees.Surname, SalesOrders.ID, SalesOrders.OrderDate
FROM SalesOrders 
JOIN Employees
ON SalesOrders.SalesRepresentative = Employees.EmployeeID;