fix: reserved keywords as column names with trailing comma detection

When trailing commas are enabled (ClickHouse/BigQuery/Snowflake), keywords
like FORMAT, SETTINGS, SAMPLE in RESERVED_FOR_COLUMN_ALIAS were incorrectly
treated as clause terminators even when used as column names.

Now checks the token after the reserved keyword: if followed by ',' or
another reserved keyword (like FROM), it's a column name, not a clause start.

Fixes: SELECT database, table, format, sample FROM t

Author: lustefaniak

src/parser/mod.rs

@@ -3519,12 +3519,20 @@ impl<'a> Parser<'a> {
                 Token::Word(ref kw)
                     if keywords::RESERVED_FOR_COLUMN_ALIAS.contains(&kw.keyword) =>
                 {
-                    // If the reserved keyword is followed by '(', it's likely a
-                    // function call (e.g., `format(...)`) not a trailing comma
-                    if self.peek_nth_token_ref(1).token == Token::LParen {
-                        false
-                    } else {
-                        true
+                    // Check the token after the reserved keyword to decide if this
+                    // is a trailing comma or a column named with a reserved word.
+                    match &self.peek_nth_token_ref(1).token {
+                        // `format(...)` - function call, not trailing comma
+                        Token::LParen => false,
+                        // `format, col2` - column name followed by more columns
+                        Token::Comma => false,
+                        // `sample FROM t` - column name followed by clause keyword
+                        Token::Word(w2)
+                            if keywords::RESERVED_FOR_COLUMN_ALIAS.contains(&w2.keyword) =>
+                        {
+                            false
+                        }
+                        _ => true,
                     }
                 }
                 Token::RParen | Token::SemiColon | Token::EOF | Token::RBracket | Token::RBrace => {

tests/sqlparser_clickhouse.rs

@@ -1622,6 +1622,8 @@ fn test_clickhouse_trailing_commas() {
         "SELECT (number, toDate('2019-05-20')), dictGetOrNull('range_key_dictionary', 'value', number, toDate('2019-05-20')), FROM system.numbers LIMIT 5 FORMAT TabSeparated",
         "SELECT (number, toDate('2019-05-20')), dictGetOrNull('range_key_dictionary', 'value', number, toDate('2019-05-20')) FROM system.numbers LIMIT 5 FORMAT TabSeparated",
     );
+    // Reserved keywords used as column names should not trigger trailing comma detection
+    clickhouse().verified_stmt("SELECT database, table, format, sample, settings FROM t");
 }
 
 #[test]