support CREATE VIEW ... UnionStmt

[XuHuaiyu]

What problem does this PR solve?

Before this:

tidb> create view v as select * from t1 union select * from t1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 39 near "union select * from t1"
tidb> create view v as (select * from t1);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 18 near "(select * from t1)"

What is changed and how it works?

After this, create view on union and create view on (select) syntax are supported.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch

[codecov]

Codecov Report

Merging #572 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #572   +/-   ##
=======================================
  Coverage   71.77%   71.77%           
=======================================
  Files          32       32           
  Lines        7820     7820           
=======================================
  Hits         5613     5613           
  Misses       1676     1676           
  Partials      531      531

Impacted Files	Coverage Δ
parser.go	70.37% <ø> (ø)	⬆️
ast/ddl.go	80.48% <100%> (ø)	⬆️

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[wshwsh12]

mysql 8.0> create view v as (select * from t union select * from t);
Query OK, 0 rows affected (0.00 sec)
tidb> create view v as (select * from t union select * from t);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 39 near "union select * from t)"

I think that create view on (union) syntax should be supported, too.

[kennytm]

Rest LGTM

[kennytm]

LGTM

[wshwsh12]

LGTM