Allow to use () as the GROUP BY nothing for PostgreSQL

Currently, PostgreSQL supports using `()` as one of the group by elements which
represent the meaning of nothing. Please refer to GROUP BY Clause
section in [PostgreSQL](https://www.postgresql.org/docs/16/sql-select.html).

This PR will close #1092.

src/parser/mod.rs

@@ -1474,6 +1474,11 @@ impl<'a> Parser<'a> {
                 let result = self.parse_comma_separated(|p| p.parse_tuple(true, true))?;
                 self.expect_token(&Token::RParen)?;
                 Ok(Expr::Rollup(result))
+            } else if self.consume_tokens(&[Token::LParen, Token::RParen]) {
+                // PostgreSQL allow to use empty tuple as a group by expression,
+                // e.g. `GROUP BY (), name`. Please refer to GROUP BY Clause section in
+                // [PostgreSQL](https://www.postgresql.org/docs/16/sql-select.html)
+                Ok(Expr::Tuple(vec![]))
             } else {
                 self.parse_expr()
             }

tests/sqlparser_postgres.rs

@@ -18,6 +18,7 @@
 mod test_utils;
 use test_utils::*;
 
+use sqlparser::ast::Expr::Identifier;
 use sqlparser::ast::*;
 use sqlparser::dialect::{GenericDialect, PostgreSqlDialect};
 use sqlparser::parser::ParserError;
@@ -4441,3 +4442,30 @@ fn test_table_unnest_with_ordinality() {
         _ => panic!("Expecting TableFactor::UNNEST with ordinality"),
     }
 }
+
+#[test]
+fn test_group_by_nothing() {
+    match pg_and_generic().verified_only_select("SELECT count(1) FROM t GROUP BY ()") {
+        Select { group_by, .. } => {
+            assert_eq!(
+                GroupByExpr::Expressions(vec![Expr::Tuple(vec![])], vec![]),
+                group_by
+            );
+        }
+    }
+
+    match pg_and_generic().verified_only_select("SELECT name, count(1) FROM t GROUP BY name, ()") {
+        Select { group_by, .. } => {
+            assert_eq!(
+                GroupByExpr::Expressions(
+                    vec![
+                        Identifier(Ident::new("name".to_string())),
+                        Expr::Tuple(vec![])
+                    ],
+                    vec![]
+                ),
+                group_by
+            );
+        }
+    }
+}