fix: preserve subquery structure when unparsing SubqueryAlias over Aggregate

When the SQL unparser encountered a SubqueryAlias node whose direct
child was an Aggregate (or other clause-building plan like Window, Sort,
Limit, Union), it would flatten the subquery into a simple table alias,
losing the aggregate entirely.

For example, a plan representing:
  SELECT j1.col FROM j1 JOIN (SELECT max(id) AS m FROM j2) AS b ON j1.id = b.m

would unparse to:
  SELECT j1.col FROM j1 INNER JOIN j2 AS b ON j1.id = b.m

dropping the MAX aggregate and the subquery.

Root cause: the SubqueryAlias handler in select_to_sql_recursively would
call subquery_alias_inner_query_and_columns (which only unwraps
Projection children) and unparse_table_scan_pushdown (which only handles
TableScan/SubqueryAlias/Projection). When both returned nothing useful
for an Aggregate child, the code recursed directly into the Aggregate,
merging its GROUP BY into the outer SELECT instead of wrapping it in a
derived subquery.

The fix adds an early check: if the SubqueryAlias's direct child is a
plan type that builds its own SELECT clauses (Aggregate, Window, Sort,
Limit, Union), emit it as a derived subquery via self.derive() with the
alias always attached, rather than falling through to the recursive
path that would flatten it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yonatan-sevenai

datafusion/sql/src/unparser/plan.rs

@@ -1085,7 +1085,7 @@ impl Unparser<'_> {
     /// must be emitted as a derived subquery `(SELECT ...) AS alias`.
     ///
     /// Plans like Aggregate or Window build their own SELECT clauses (GROUP BY,
-    /// window functions). 
+    /// window functions).
     fn requires_derived_subquery(plan: &LogicalPlan) -> bool {
         matches!(
             plan,

datafusion/sql/tests/cases/plan_to_sql.rs

@@ -2916,7 +2916,10 @@ fn test_unparse_manual_join_with_subquery_aggregate() -> Result<()> {
     // Build the right side: SELECT max(j2_id) AS max_id FROM j2
     let right_scan = table_scan(Some("j2"), &j2_schema, None)?.build()?;
     let right_agg = LogicalPlanBuilder::from(right_scan)
-        .aggregate(vec![] as Vec<Expr>, vec![max(col("j2.j2_id")).alias("max_id")])?
+        .aggregate(
+            vec![] as Vec<Expr>,
+            vec![max(col("j2.j2_id")).alias("max_id")],
+        )?
         .build()?;
     let right_subquery = subquery_alias(right_agg, "b")?;