tidb reports syntax error while mysql does not

[eurekaka]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t(a int);
-- SQL_1
SELECT t1.a AS a FROM (SELECT a FROM t) AS t1;
-- SQL_2
SELECT t1.a AS a FROM ((SELECT a FROM t) AS t1);

2. What did you expect to see?

As MySQL, there should be no syntax error for SQL_1 and SQL_2.

3. What did you see instead?

SQL_1 successes, while SQL_2 fails.

4. What version of TiDB SQL Parser are you using?

3abb698

I tried to change the SetOprStmt1 in line 8021 back to SelectStmt, the above problem can be fixed, but there would be some test failures for set operations.

 8010 TableFactor:
 8011     TableName PartitionNameListOpt TableAsNameOpt IndexHintListOpt TableSampleOpt
 8012     {
 8013         tn := $1.(*ast.TableName)
 8014         tn.PartitionNames = $2.([]model.CIStr)
 8015         tn.IndexHints = $4.([]*ast.IndexHint)
 8016         if $5 != nil {
 8017             tn.TableSample = $5.(*ast.TableSample)
 8018         }
 8019         $$ = &ast.TableSource{Source: tn, AsName: $3.(model.CIStr)}
 8020     }
 8021 |   '(' SetOprStmt1 ')' TableAsNameOpt
 8022     {
 8023         if st, isSel := $2.(*ast.SelectStmt); isSel {
 8024             endOffset := parser.endOffset(&yyS[yypt-1])
 8025             parser.setLastSelectFieldText(st, endOffset)
 8026         }
 8027         $$ = &ast.TableSource{Source: $2.(ast.ResultSetNode), AsName: $4.(model.CIStr)}
 8028     }
 8029 |   '(' TableRefs ')'
 8030     {
 8031         $$ = $2
 8032     }

[xiongjiwei]

MySQL 8.0 will return syntax error too

mysql> SELECT t1.a AS a FROM ((SELECT a FROM t) AS t1);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 1
mysql> select version();
+-------------------------+
| version()               |
+-------------------------+
| 8.0.22-0ubuntu0.20.04.3 |
+-------------------------+
1 row in set (0.00 sec)

[eurekaka]

The current Restore() functions in parser may generate queries like SQL_2, which would be problematic then.

[xiongjiwei]

The current Restore() functions in parser may generate queries like SQL_2, which would be problematic then.

can you please give some examples?

[xiongjiwei]

select * from ((select a from t) t1 join t t2); # invalid in tidb. valid in MySQL
SELECT * FROM ((SELECT a FROM t) t1);   # invalid both tidb and MySQL