SQLite, DuckDB: Support aggregate ORDER BY without arguments

src/dialect/duckdb.rs

@@ -57,6 +57,10 @@ impl Dialect for DuckDbDialect {
         true
     }
 
+    fn supports_aggregate_order_by_with_empty_args(&self) -> bool {
+        true
+    }
+
     fn supports_named_fn_args_with_eq_operator(&self) -> bool {
         true
     }

src/dialect/generic.rs

@@ -173,6 +173,10 @@ impl Dialect for GenericDialect {
         true
     }
 
+    fn supports_aggregate_order_by_with_empty_args(&self) -> bool {
+        true
+    }
+
     fn supports_comment_on(&self) -> bool {
         true
     }

src/dialect/mod.rs

@@ -2038,6 +2038,15 @@ pub trait Dialect: Debug + Any {
     fn supports_cast_empty_data_type_to_unspecified(&self) -> bool {
         false
     }
+
+    /// Returns true if the dialect allows an `ORDER BY` clause inside an aggregate function
+    /// call with no preceding arguments, e.g. `count(ORDER BY a)`.
+    ///
+    /// [SQLite](https://www.sqlite.org/lang_aggfunc.html#aggorderby),
+    /// [DuckDB](https://duckdb.org/docs/current/sql/functions/aggregates#order-by-clause-in-aggregate-functions)
+    fn supports_aggregate_order_by_with_empty_args(&self) -> bool {
+        false
+    }
 }
 
 /// Operators for which precedence must be defined.

src/dialect/sqlite.rs

@@ -146,4 +146,8 @@ impl Dialect for SQLiteDialect {
     fn supports_parens_around_table_factor(&self) -> bool {
         true
     }
+
+    fn supports_aggregate_order_by_with_empty_args(&self) -> bool {
+        true
+    }
 }

src/parser/mod.rs

@@ -19345,7 +19345,14 @@ impl<'a> Parser<'a> {
         }
 
         let duplicate_treatment = self.parse_duplicate_treatment()?;
-        let args = self.parse_comma_separated(Parser::parse_function_args)?;
+        let args = if duplicate_treatment != Some(DuplicateTreatment::Distinct)
+            && self.dialect.supports_aggregate_order_by_with_empty_args()
+            && self.peek_keywords(&[Keyword::ORDER, Keyword::BY])
+        {
+            vec![]
+        } else {
+            self.parse_comma_separated(Parser::parse_function_args)?
+        };
 
         if self.parse_keyword(Keyword::WHERE) {
             clauses.push(FunctionArgumentClause::Where(self.parse_expr()?));

tests/sqlparser_common.rs

@@ -20707,3 +20707,25 @@ fn parse_string_literal_alias() {
     dialects.verified_stmt("SELECT 1 AS 'x'");
     dialects.verified_stmt("SELECT a FROM t AS 'y'");
 }
+
+#[test]
+fn parse_aggregate_order_by_with_empty_args() {
+    let dialects = all_dialects_where(|d| d.supports_aggregate_order_by_with_empty_args());
+    dialects.verified_stmt("SELECT count(ORDER BY a) FROM t");
+    dialects.verified_stmt("SELECT count(ORDER BY a ASC, b DESC) FROM t");
+    dialects.verified_stmt("SELECT count(ALL ORDER BY a) FROM t");
+    assert_eq!(
+        dialects.parse_sql_statements("SELECT count(DISTINCT ORDER BY a) FROM t"),
+        Err(ParserError::ParserError(
+            "Expected: ), found: BY".to_string()
+        ))
+    );
+
+    assert_eq!(
+        all_dialects_where(|d| !d.supports_aggregate_order_by_with_empty_args())
+            .parse_sql_statements("SELECT count(ORDER BY a) FROM t"),
+        Err(ParserError::ParserError(
+            "Expected: ), found: BY".to_string()
+        ))
+    );
+}