Fix(optimizer): compute external columns for union sopes correctly (#…

…2864)

sqlglot/optimizer/scope.py

@@ -323,9 +323,14 @@ def external_columns(self):
                 sources in the current scope.
         """
         if self._external_columns is None:
-            self._external_columns = [
-                c for c in self.columns if c.table not in self.selected_sources
-            ]
+            if isinstance(self.expression, exp.Union):
+                left, right = self.union_scopes
+                self._external_columns = left.external_columns + right.external_columns
+            else:
+                self._external_columns = [
+                    c for c in self.columns if c.table not in self.selected_sources
+                ]
+
         return self._external_columns
 
     @property

tests/fixtures/optimizer/qualify_columns.sql

@@ -230,6 +230,11 @@ SELECT (SELECT y.c AS c FROM y AS y) AS _col_0 FROM x AS x;
 SELECT a FROM (SELECT a FROM x) WHERE a IN (SELECT b FROM (SELECT b FROM y));
 SELECT _q_1.a AS a FROM (SELECT x.a AS a FROM x AS x) AS _q_1 WHERE _q_1.a IN (SELECT _q_0.b AS b FROM (SELECT y.b AS b FROM y AS y) AS _q_0);
 
+# dialect: mysql
+# execute: false
+SELECT * FROM table_a as A WHERE A.col1 IN (SELECT MAX(B.col2) FROM table_b as B UNION ALL SELECT MAX(C.col2) FROM table_b as C);
+SELECT * FROM table_a AS `A` WHERE `A`.col1 IN (SELECT MAX(`B`.col2) AS _col_0 FROM table_b AS `B` UNION ALL SELECT MAX(`C`.col2) AS _col_0 FROM table_b AS `C`);
+
 --------------------------------------
 -- Correlated subqueries
 --------------------------------------