Don't add an alias to subqueries

docs/pages/docs/Advanced Features/isolates.md

@@ -43,8 +43,11 @@ import 'package:moor/isolate.dart';
 
 // This needs to be a top-level method because it's run on a background isolate
 DatabaseConnection _backgroundConnection() {
-    // construct the database. You can also wrap the VmDatabase in a "LazyDatabase" if you need to run
-    // work before the database opens.
+    // Construct the database to use. This example uses a non-persistent in-memory database each
+    // time. You can use your existing VmDatabase with a file as well, or a `LazyDatabase` if you
+    // need to construct it asynchronously.
+    // When using a Flutter plugin like `path_provider` to determine the path, also see the
+    // "Initialization on the main thread" section below!
     final database = VmDatabase.memory();
     return DatabaseConnection.fromExecutor(database);
 }

moor_generator/lib/src/analyzer/sql_queries/explicit_alias_transformer.dart

@@ -1,14 +1,24 @@
 import 'package:sqlparser/sqlparser.dart';
 
+/// A transformer adding explicit aliases to columns in a projection.
+///
+/// In sqlite3, the result name of columns without an alias is undefined. While
+/// the names of direct column references (`SELECT foo FROM bar`) is unlikely
+/// to change, we shouldn't assume that for more complex columns (`SELECT
+/// MAX(id) * 14 FROM bar`). This transformer adds an alias to such columns
+/// which avoids undefined behavior that might be different across sqlite3
+/// versions.
 class ExplicitAliasTransformer extends Transformer<bool> {
   int _aliasCounter = 0;
   final Map<Expression, String> _renamed = {};
 
+  /// Rewrites an SQL [node] to use explicit aliases for columns.
   AstNode rewrite(AstNode node) {
     node = transform(node, true)!;
     return _PatchReferences(this).transform(node, null)!;
   }
 
+  /// Obtain the new name for a [column] after an alias has been added.
   String? newNameFor(Column column) {
     while (column is CompoundSelectColumn) {
       // In compound select statement, the first column determines the overall
@@ -55,6 +65,14 @@ class ExplicitAliasTransformer extends Transformer<bool> {
       return super.visitExpressionResultColumn(e, arg);
     }
   }
+
+  @override
+  AstNode? visitSubQuery(SubQuery e, bool arg) {
+    // Subquery expressions only have a single column, so the inner column
+    // doesn't matter. For instance, `SELECT (SELECT 1) AS foo` has no undefined
+    // behavior, even though the inner `1` has no alias.
+    return e..transformChildren(this, false);
+  }
 }
 
 class _PatchReferences extends Transformer<void> {

moor_generator/test/analyzer/sql_queries/explicit_alias_transformer_test.dart

@@ -30,6 +30,10 @@ void main() {
         'SELECT _c0 FROM (SELECT 1 + 2 AS _c0)');
   });
 
+  test('does not rewrite subquery expressions', () {
+    _test('SELECT (SELECT 1)', 'SELECT (SELECT 1) AS _c0');
+  });
+
   test('rewrites compound select statements', () {
     _test("SELECT 1 + 2, 'foo' UNION ALL SELECT 3+ 4, 'bar'",
         "SELECT 1 + 2 AS _c0, 'foo' AS _c1 UNION ALL SELECT 3 + 4, 'bar'");