Join Correlation Name

When joining tables with USING, the listed columns are merged and no
longer belong to either the left or the right side.  That means they can
no longer be qualified which can often be an inconvenience.

SELECT a.x, b.y, z FROM a INNER JOIN b USING (z);

The SQL standard provides a workaround for this by allowing an alias on
the join clause. (<join correlation name> in section 7.10)

SELECT j.x, j.y, j.z FROM a INNER JOIN b USING (z) AS j;

Attached is a patch (based on 517bf2d910) adding this feature.

--

Vik Fearing