Github user hvanhovell commented on a diff in the pull request:
https://github.com/apache/spark/pull/13534#discussion_r66067691
--- Diff:
sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4 ---
@@ -594,16 +594,13 @@ qualifiedName
: identifier ('.' identifier)*
;
-// Identifier that also allows the use of a number of SQL keywords (mainly
for backwards compatibility).
-looseIdentifier
- : identifier
- | FROM
- | TO
- | TABLE
- | WITH
+identifier
+ : strictIdentifier
+ | ANTI | FULL | INNER | LEFT | SEMI | RIGHT | NATURAL | UNION |
INTERSECT | EXCEPT | DATABASE | ON | JOIN | CROSS
+ | SELECT | FROM | WHERE | HAVING | FROM | TO | TABLE | WITH | NOT
--- End diff --
I don't have a reference but let me explain. The general issue is if we
would make them non-reserved they would introduce ambiguities during the
parsing (if you are lucky you get a parsing error or if you are unlucky very
weird behavior).
I can give you an example for joins: `select * from a left join b on b.id =
a.id` would be interpreted as *a* **aliased as left** *inner join b using
condition b.id = a.id*.
We might not need the other keywords here. Let me check if I can move them.
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