Support FILTER in over clause (#1007)

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

src/ast/mod.rs

@@ -1070,8 +1070,11 @@ impl Display for WindowType {
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 #[cfg_attr(feature = "visitor", derive(Visit, VisitMut))]
 pub struct WindowSpec {
+    /// `OVER (PARTITION BY ...)`
     pub partition_by: Vec<Expr>,
+    /// `OVER (ORDER BY ...)`
     pub order_by: Vec<OrderByExpr>,
+    /// `OVER (window frame)`
     pub window_frame: Option<WindowFrame>,
 }
 
@@ -3729,6 +3732,8 @@ impl fmt::Display for CloseCursor {
 pub struct Function {
     pub name: ObjectName,
     pub args: Vec<FunctionArg>,
+    /// e.g. `x > 5` in `COUNT(x) FILTER (WHERE x > 5)`
+    pub filter: Option<Box<Expr>>,
     pub over: Option<WindowType>,
     // aggregate functions may specify eg `COUNT(DISTINCT x)`
     pub distinct: bool,
@@ -3777,6 +3782,10 @@ impl fmt::Display for Function {
                 display_comma_separated(&self.order_by),
             )?;
 
+            if let Some(filter_cond) = &self.filter {
+                write!(f, " FILTER (WHERE {filter_cond})")?;
+            }
+
             if let Some(o) = &self.over {
                 write!(f, " OVER {o}")?;
             }

src/ast/visitor.rs

@@ -506,7 +506,7 @@ where
 ///     *expr = Expr::Function(Function {
 ///           name: ObjectName(vec![Ident::new("f")]),
 ///           args: vec![FunctionArg::Unnamed(FunctionArgExpr::Expr(old_expr))],
-///           over: None, distinct: false, special: false, order_by: vec![],
+///           filter: None, over: None, distinct: false, special: false, order_by: vec![],
 ///      });
 ///   }
 ///   ControlFlow::<()>::Continue(())

src/dialect/sqlite.rs

@@ -35,6 +35,10 @@ impl Dialect for SQLiteDialect {
             || ('\u{007f}'..='\u{ffff}').contains(&ch)
     }
 
+    fn supports_filter_during_aggregation(&self) -> bool {
+        true
+    }
+
     fn is_identifier_part(&self, ch: char) -> bool {
         self.is_identifier_start(ch) || ch.is_ascii_digit()
     }

src/parser/mod.rs

@@ -772,6 +772,7 @@ impl<'a> Parser<'a> {
                     Ok(Expr::Function(Function {
                         name: ObjectName(vec![w.to_ident()]),
                         args: vec![],
+                        filter: None,
                         over: None,
                         distinct: false,
                         special: true,
@@ -957,6 +958,17 @@ impl<'a> Parser<'a> {
         self.expect_token(&Token::LParen)?;
         let distinct = self.parse_all_or_distinct()?.is_some();
         let (args, order_by) = self.parse_optional_args_with_orderby()?;
+        let filter = if self.dialect.supports_filter_during_aggregation()
+            && self.parse_keyword(Keyword::FILTER)
+            && self.consume_token(&Token::LParen)
+            && self.parse_keyword(Keyword::WHERE)
+        {
+            let filter = Some(Box::new(self.parse_expr()?));
+            self.expect_token(&Token::RParen)?;
+            filter
+        } else {
+            None
+        };
         let over = if self.parse_keyword(Keyword::OVER) {
             if self.consume_token(&Token::LParen) {
                 let window_spec = self.parse_window_spec()?;
@@ -970,6 +982,7 @@ impl<'a> Parser<'a> {
         Ok(Expr::Function(Function {
             name,
             args,
+            filter,
             over,
             distinct,
             special: false,
@@ -987,6 +1000,7 @@ impl<'a> Parser<'a> {
         Ok(Expr::Function(Function {
             name,
             args,
+            filter: None,
             over: None,
             distinct: false,
             special,