Don't silently accept naked OUTER JOINS

`SELECT * FROM a OUTER JOIN b` was previously being parsed as an inner
join where table `a` was aliased to `OUTER`. This is extremely
surprising, as the user likely intended to say FULL OUTER JOIN. Since
the SQL specification lists OUTER as a keyword, we are well within our
rights to return an error here.

src/dialect/keywords.rs

@@ -422,7 +422,12 @@ pub const RESERVED_FOR_TABLE_ALIAS: &[&str] = &[
     // Reserved as both a table and a column alias:
     WITH, SELECT, WHERE, GROUP, HAVING, ORDER, LIMIT, OFFSET, FETCH, UNION, EXCEPT, INTERSECT,
     // Reserved only as a table alias in the `FROM`/`JOIN` clauses:
-    ON, JOIN, INNER, CROSS, FULL, LEFT, RIGHT, NATURAL, USING,
+    ON, JOIN, INNER, CROSS, FULL, LEFT, RIGHT, NATURAL, USING, LIMIT, OFFSET, FETCH,
+    // Reserved not because of ambiguity, but so that parsing `SELECT * FROM a
+    // OUTER JOIN b` causes a syntax error, rather than silently parsing to an
+    // inner join where table `a` is aliased as `OUTER`, which is certainly not
+    // what the user intended and also not valid according to the SQL standard.
+    OUTER,
 ];
 
 /// Can't be used as a column alias, so that `SELECT <expr> alias`

src/sqlparser.rs

@@ -1648,6 +1648,7 @@ impl Parser {
                             _ => unreachable!(),
                         }
                     }
+                    "OUTER" => return self.expected("LEFT, RIGHT, or FULL", self.peek_token()),
                     _ if natural => {
                         return self.expected("a join type after NATURAL", self.peek_token());
                     }

tests/sqlparser_common.rs

@@ -1801,6 +1801,12 @@ fn parse_join_syntax_variants() {
         "SELECT c1 FROM t1 FULL OUTER JOIN t2 USING(c1)",
         "SELECT c1 FROM t1 FULL JOIN t2 USING(c1)",
     );
+
+    let res = parse_sql_statements("SELECT * FROM a OUTER JOIN b ON 1");
+    assert_eq!(
+        ParserError::ParserError("Expected LEFT, RIGHT, or FULL, found: OUTER".to_string()),
+        res.unwrap_err()
+    );
 }
 
 #[test]