When both of the two table expressions being joined do not contain commas, SQL Anywhere examines the foreign key relationships in the pairs of tables in the statement, and generates a single join condition.
For example, the following join has two table-pairs, A-C and B-C.
(A NATURAL JOIN B) KEY JOIN C
SQL Anywhere generates a single join condition for joining C with
(A NATURAL JOIN B) by looking at the foreign key relationships within the table-pairs A-C and B-C. It generates one join condition for the two
pairs according to the rules for determining key joins when there are multiple foreign key relationships:
First, it looks at both A-C and B-C for a single foreign key that has the same role name as the correlation name of one of the primary key tables it references. If there is exactly one foreign key meeting this criterion, it uses it. If there is more than one foreign key with the same role name as the correlation name of a table, the join is considered to be ambiguous and an error is issued.
If there is no foreign key with the same name as the correlation name of a table, SQL Anywhere looks for any foreign key relationship between the tables. If there is one, it uses it. If there is more than one, the join is considered to be ambiguous and an error is issued.
If there is no foreign key relationship, an error is issued.
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