feat: update array_agg to use COALESCE for handling empty results in DuckDB and SQLite

Author: sunyeongchoi

prqlc/prqlc/src/sql/std.sql.prql

@@ -275,6 +275,9 @@ module clickhouse {
 }
 
 module duckdb {
+  @{window_frame=true, coalesce="[]"}
+  let array_agg = column -> s"ARRAY_AGG({column:0})"
+
   @{window_frame=true}
   let max_by = column by -> s"arg_max({column:0}, {by:0})"
 
@@ -439,7 +442,7 @@ module sqlite {
   @{window_frame=true, coalesce="''"}
   let concat_array = column -> s"GROUP_CONCAT({column:0}, '')"
 
-  @{window_frame=true}
+  @{window_frame=true, coalesce="'[]'"}
   let array_agg = column -> s"json_group_array({column:0})"
 
   @{binding_strength=11}

prqlc/prqlc/tests/integration/snapshots/integration__queries__compileall__aggregation.snap

@@ -4,6 +4,20 @@ expression: "# mysql:skip\n# clickhouse:skip\n# glaredb:skip (the string_agg fun
 input_file: prqlc/prqlc/tests/integration/queries/aggregation.prql
 ---
 
+--- generic
++++ duckdb
+@@ -1,10 +1,10 @@
+ SELECT
+   COALESCE(SUM(track_id), 0),
+   COALESCE(STRING_AGG(name, ''), ''),
+-  ARRAY_AGG(name),
++  COALESCE(ARRAY_AGG(name), []),
+   COALESCE(BOOL_AND(name = ''), TRUE),
+   COALESCE(BOOL_OR(name = ''), FALSE)
+ FROM
+   tracks
+ WHERE
+   genre_id = 100
 
 
 
@@ -18,7 +32,7 @@ input_file: prqlc/prqlc/tests/integration/queries/aggregation.prql
 -  COALESCE(BOOL_AND(name = ''), TRUE),
 -  COALESCE(BOOL_OR(name = ''), FALSE)
 +  COALESCE(GROUP_CONCAT(name, ''), ''),
-+  json_group_array(name),
++  COALESCE(json_group_array(name), '[]'),
 +  COALESCE(MIN(name = '') > 0, TRUE),
 +  COALESCE(MAX(name = '') > 0, FALSE)
  FROM

prqlc/prqlc/tests/integration/sql.rs

@@ -52,7 +52,7 @@ fn test_array_agg_per_dialect() {
 
     assert_snapshot!(compile_with_sql_dialect(query, sql::Dialect::DuckDb).unwrap(), @r#"
     SELECT
-      ARRAY_AGG(name) AS names
+      COALESCE(ARRAY_AGG(name), []) AS names
     FROM
       tracks
     "#);
@@ -73,7 +73,7 @@ fn test_array_agg_per_dialect() {
 
     assert_snapshot!(compile_with_sql_dialect(query, sql::Dialect::SQLite).unwrap(), @r#"
     SELECT
-      json_group_array(name) AS names
+      COALESCE(json_group_array(name), '[]') AS names
     FROM
       tracks
     "#);