When there is a multiple-table expression on at least one side of a natural join, SQL Anywhere generates a join condition by comparing the set of columns for each side of the join operator, and looking for columns that have the same name.
For example, in the statement
SELECT * FROM (A JOIN B) NATURAL JOIN (C JOIN D);
there are two table expressions. The column names in the table expression
A JOIN B are compared to the column names in the table expression
C JOIN D, and a join condition is generated for each unambiguous pair of matching column names. An unambiguous pair of matching columns means that the column name occurs in both table expressions, but does not occur twice in the same table expression.
If there is a pair of ambiguous column names, an error is issued. However, a column name may occur twice in the same table expression, as long as it doesn't also match the name of a column in the other table expression.
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