Support WITH clause for inner select statements

sqlparser/lib/src/ast/expressions/subquery.dart

@@ -3,7 +3,7 @@ part of '../ast.dart';
 /// A subquery, which is an expression. It is expected that the inner query
 /// only returns one column and one row.
 class SubQuery extends Expression {
-  final SelectStatement select;
+  final BaseSelectStatement select;
 
   SubQuery({this.select});
 
@@ -18,7 +18,7 @@ class SubQuery extends Expression {
 }
 
 class ExistsExpression extends Expression {
-  final SelectStatement select;
+  final BaseSelectStatement select;
 
   ExistsExpression({@required this.select});
 

sqlparser/lib/src/reader/parser/crud.dart

@@ -66,11 +66,8 @@ mixin CrudParser on ParserBase {
       ..withToken = withToken;
   }
 
-  /// Parses a select statement as defined in [the sqlite documentation][s-d],
-  /// which means that compound selects and a with clause is supported.
-  ///
-  /// [s-d]: https://sqlite.org/syntax/select-stmt.html
-  BaseSelectStatement _defaultSelect() {
+  @override
+  BaseSelectStatement _fullSelect() {
     final clause = _withClause();
     return select(withClause: clause);
   }
@@ -82,6 +79,17 @@ mixin CrudParser on ParserBase {
     } else {
       final firstTokenOfBase = _peek;
       final first = _selectNoCompound(withClause);
+
+      if (first == null) {
+        // _selectNoCompound returns null if there's no select statement at the
+        // current position. That's fine if we didn't encounter an with clause
+        // already
+        if (withClause != null) {
+          _error('Expected a SELECT statement to follow the WITH clause here');
+        }
+        return null;
+      }
+
       final parts = <CompoundSelectPart>[];
 
       while (true) {
@@ -631,7 +639,7 @@ mixin CrudParser on ParserBase {
       return const DefaultValues();
     } else {
       return SelectInsertSource(
-          _defaultSelect() ?? _error('Expeced a select statement'));
+          _fullSelect() ?? _error('Expeced a select statement'));
     }
   }
 

sqlparser/lib/src/reader/parser/expressions.dart

@@ -194,7 +194,7 @@ mixin ExpressionParser on ParserBase {
       final existsToken = _previous;
       _consume(
           TokenType.leftParen, 'Expected opening parenthesis after EXISTS');
-      final selectStmt = select(noCompound: true) as SelectStatement;
+      final selectStmt = _fullSelect() ?? _error('Expected a select statement');
       _consume(TokenType.rightParen,
           'Expected closing paranthesis to finish EXISTS expression');
       return ExistsExpression(select: selectStmt)
@@ -263,7 +263,9 @@ mixin ExpressionParser on ParserBase {
 
     if (_matchOne(TokenType.leftParen)) {
       final left = _previous;
-      if (_peek.type == TokenType.select) {
+
+      final selectStmt = _fullSelect(); // returns null if there's no select
+      if (selectStmt != null) {
         final stmt = select(noCompound: true) as SelectStatement;
         _consume(TokenType.rightParen, 'Expected a closing bracket');
         return SubQuery(select: stmt)..setSpan(left, _previous);
@@ -380,7 +382,7 @@ mixin ExpressionParser on ParserBase {
         _consume(TokenType.leftParen, 'Expected opening parenthesis for tuple');
     final expressions = <Expression>[];
 
-    final subQuery = select(noCompound: true) as SelectStatement;
+    final subQuery = _fullSelect();
     if (subQuery == null) {
       // no sub query found. read expressions that form the tuple.
       // tuples can be empty `()`, so only start parsing values when it's not

sqlparser/lib/src/reader/parser/parser.dart

@@ -169,10 +169,19 @@ abstract class ParserBase {
   /// [CompoundSelectStatement]. If [noCompound] is set to true, the parser will
   /// only attempt to parse a [SelectStatement].
   ///
+  /// This method doesn't parse WITH clauses, most users would probably want to
+  /// use [_fullSelect] instead.
+  ///
   /// See also:
   /// https://www.sqlite.org/lang_select.html
   BaseSelectStatement select({bool noCompound});
 
+  /// Parses a select statement as defined in [the sqlite documentation][s-d],
+  /// which means that compound selects and a with clause is supported.
+  ///
+  /// [s-d]: https://sqlite.org/syntax/select-stmt.html
+  BaseSelectStatement _fullSelect();
+
   Literal _literalOrNull();
   OrderingMode _orderingModeOrNull();