Submitted by: @mrotteveel

Attachments:
query-expr-v3.diff

See also discussion with subject "Standard-compliance for query expressions" on firebird-devel (March 6th, 2021).

The SQL standard allows parentheses around query expressions (without with-clause), and Firebird does not. It would be helpful if Firebird added this support.

Specifically, SQL:2016 specifies the following:

```
<query expression> ::=
[ <with clause> ] <query expression body>
[ <order by clause> ] [ <result offset clause> ] [ <fetch first clause> ]

<query expression body> ::=
<query term>
| <query expression body> UNION [ ALL | DISTINCT ]
[ <corresponding spec> ] <query term>
| <query expression body> EXCEPT [ ALL | DISTINCT ]
[ <corresponding spec> ] <query term>

<query term> ::=
<query primary>
| <query term> INTERSECT [ ALL | DISTINCT ]
[ <corresponding spec> ] <query primary>

<query primary> ::=
<simple table>
| <left paren> <query expression body>
[ <order by clause> ] [ <result offset clause> ] [ <fetch first clause> ]
<right paren>

<simple table> ::=
<query specification>
| <table value constructor>
| <explicit table>

<query specification> ::=
SELECT [ <set quantifier> ] <select list> <table expression>

::= \[ \] \[ \] \[ \] \[ \] \`\`\`

If I follow the grammar in parse.y correctly, the problem is that in Firebird, <query primary> is basically <query specification>, so it's missing the following alternative:

```
<left paren> <query expression body>
[ <order by clause> ] [ <result offset clause> ] [ <fetch first clause> ]
<right paren>
```

Adriano wrote the attached patch.