Josh Rosen created SPARK-20841:
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Summary: Support table column aliases in FROM clause
Key: SPARK-20841
URL: https://issues.apache.org/jira/browse/SPARK-20841
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 2.0.0
Reporter: Josh Rosen
Priority: Minor
Some SQL dialects support a relatively obscure "table column aliases" feature
where you can rename columns when aliasing a relation in a {{FROM}} clause. For
example:
{code}
SELECT * FROM onecolumn AS a(x) JOIN onecolumn AS b(y) ON a.x = b.y
{code}
Spark does not currently support this. I would like to add support for this in
order to allow me to run a corpus of existing queries which depend on this
syntax.
There's a good writeup on this at
http://modern-sql.com/feature/table-column-aliases, which has additional
examples and describes other databases' degrees of support for this feature.
One tricky thing to figure out will be whether FROM clause column aliases take
precedence over aliases in the SELECT clause. When adding support for this, we
should make sure to add sufficient testing of several corner-cases, including:
* Aliasing in both the SELECT and FROM clause
* Aliasing columns in the FROM clause both with and without an explicit AS.
* Aliasing the wrong number of columns in the FROM clause, both greater and
fewer columns than were selected in the SELECT clause.
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