Stricter parsing for subqueries (3/4)

This makes the parser more strict when handling SELECTs nested
somewhere in the main statement:

1) instead of accepting SELECT anywhere in the expression where an
   operand was expected, we only accept it inside parens. (I've added a
   test for the currently supported syntax, <scalar subquery> in ANSI
   SQL terms)

2) instead of accepting any expression in the derived table context:
   `FROM ( ... )` - we only look for a SELECT subquery there.

Due to #1, I had to swith the 'ansi' test from invoking the expression
parser to the statement parser.

src/sqlast/mod.rs

@@ -77,8 +77,9 @@ pub enum ASTNode {
         relation: Box<ASTNode>, // SQLNested or SQLCompoundIdentifier
         alias: Option<SQLIdent>,
     },
-    /// SELECT
-    SQLSelect(SQLSelect),
+    /// A parenthesized subquery `(SELECT ...)`, used in expression like
+    /// `SELECT (subquery) AS x` or `WHERE (subquery) = x`
+    SQLSubquery(SQLSelect),
 }
 
 impl ToString for ASTNode {
@@ -139,7 +140,7 @@ impl ToString for ASTNode {
                     relation.to_string()
                 }
             }
-            ASTNode::SQLSelect(s) => s.to_string(),
+            ASTNode::SQLSubquery(s) => format!("({})", s.to_string()),
         }
     }
 }

src/sqlparser.rs

@@ -158,7 +158,6 @@ impl Parser {
         match self.next_token() {
             Some(t) => match t {
                 Token::SQLWord(w) => match w.keyword.as_ref() {
-                    "SELECT" => Ok(ASTNode::SQLSelect(self.parse_select()?)),
                     "TRUE" | "FALSE" | "NULL" => {
                         self.prev_token();
                         self.parse_sql_value()
@@ -197,9 +196,13 @@ impl Parser {
                     self.parse_sql_value()
                 }
                 Token::LParen => {
-                    let expr = self.parse_expr()?;
+                    let expr = if self.parse_keyword("SELECT") {
+                        ASTNode::SQLSubquery(self.parse_select()?)
+                    } else {
+                        ASTNode::SQLNested(Box::new(self.parse_expr()?))
+                    };
                     self.expect_token(&Token::RParen)?;
-                    Ok(ASTNode::SQLNested(Box::new(expr)))
+                    Ok(expr)
                 }
                 _ => parser_err!(format!(
                     "Prefix parser expected a keyword but found {:?}",
@@ -1184,8 +1187,10 @@ impl Parser {
     /// A table name or a parenthesized subquery, followed by optional `[AS] alias`
     pub fn parse_table_factor(&mut self) -> Result<ASTNode, ParserError> {
         let relation = if self.consume_token(&Token::LParen) {
-            self.prev_token();
-            self.parse_subexpr(0)? /* TBD (3) */
+            self.expect_keyword("SELECT")?;
+            let subquery = self.parse_select()?;
+            self.expect_token(&Token::RParen)?;
+            ASTNode::SQLSubquery(subquery)
         } else {
             self.parse_compound_identifier(&Token::Period)?
         };

tests/sqlparser_ansi.rs

@@ -4,25 +4,16 @@ extern crate sqlparser;
 use sqlparser::dialect::AnsiSqlDialect;
 use sqlparser::sqlast::*;
 use sqlparser::sqlparser::*;
-use sqlparser::sqltokenizer::*;
 
 #[test]
 fn parse_simple_select() {
     let sql = String::from("SELECT id, fname, lname FROM customer WHERE id = 1");
-    let ast = parse_sql_expr(&sql);
-    match ast {
-        ASTNode::SQLSelect(SQLSelect { projection, .. }) => {
+    let ast = Parser::parse_sql(&AnsiSqlDialect {}, sql).unwrap();
+    assert_eq!(1, ast.len());
+    match ast.first().unwrap() {
+        SQLStatement::SQLSelect(SQLSelect { projection, .. }) => {
             assert_eq!(3, projection.len());
         }
         _ => assert!(false),
     }
 }
-
-fn parse_sql_expr(sql: &str) -> ASTNode {
-    let dialect = AnsiSqlDialect {};
-    let mut tokenizer = Tokenizer::new(&dialect, &sql);
-    let tokens = tokenizer.tokenize().unwrap();
-    let mut parser = Parser::new(tokens);
-    let ast = parser.parse_expr().unwrap();
-    ast
-}

tests/sqlparser_generic.rs

@@ -664,6 +664,13 @@ fn parse_join_syntax_variants() {
     );
 }
 
+#[test]
+fn parse_derived_tables() {
+    let sql = "SELECT a.x, b.y FROM (SELECT x FROM foo) AS a CROSS JOIN (SELECT y FROM bar) AS b";
+    let _ = verified_only_select(sql);
+    //TODO: add assertions
+}
+
 #[test]
 fn parse_multiple_statements() {
     fn test_with(sql1: &str, sql2_kw: &str, sql2_rest: &str) {
@@ -695,6 +702,29 @@ fn parse_multiple_statements() {
     assert_eq!(0, res.unwrap().len());
 }
 
+#[test]
+fn parse_scalar_subqueries() {
+    use self::ASTNode::*;
+    let sql = "(SELECT 1) + (SELECT 2)";
+    match verified_expr(sql) {
+        SQLBinaryExpr {
+            op: SQLOperator::Plus, ..
+            //left: box SQLSubquery { .. },
+            //right: box SQLSubquery { .. },
+        } => assert!(true),
+        _ => assert!(false),
+    };
+}
+
+#[test]
+fn parse_invalid_subquery_without_parens() {
+    let res = parse_sql_statements("SELECT SELECT 1 FROM bar WHERE 1=1 FROM baz");
+    assert_eq!(
+        ParserError::ParserError("Expected end of statement, found: 1".to_string()),
+        res.unwrap_err()
+    );
+}
+
 fn only<'a, T>(v: &'a Vec<T>) -> &'a T {
     assert_eq!(1, v.len());
     v.first().unwrap()