the verbosity of "(SELECT * FROM A)"?

Question

This might be a novice question. But sometimes, I find it necessary to use things like (SELECT * FROM A) in doing relational algebra in SQL (instead of using the relation A itself).

For example, when trying to work on the UNION of two relations A and B having the same structure, the following

SELECT * FROM (A UNION B) t;

generates a syntax error (with PostgreSQL 9.x).

ERROR:  syntax error at or near "UNION"

And I had to do

SELECT * FROM ((SELECT * FROM A) UNION (SELECT * FROM B)) t;

My questions are:

Is the verbosity of the subquery (SELECT * FROM A) really necessary in the SQL language (or am I missing something about better way to write it)? Why can't the relations A and B be used directly here?

Is (SELECT * FROM A) equivalent to A in relational algebra? What does the expansion (SELECT * FROM A) buy us? When is the long form required?

Answer 1

If you want shorter syntax in SQL, there is the TABLE name syntax that you can use - where name is a table name (base table or view or cte):

SELECT * FROM ((TABLE A) UNION (TABLE B)) t ;

The parentheses are not needed:

SELECT * FROM (TABLE A UNION TABLE B) t ;

and you could even write:

TABLE A UNION TABLE B ;

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Regarding whether the syntax is ISO/ANSI SQL, I found this in a (copy of a) draft of SQL 2011:

6.41 <multiset value constructor>

Function

Specify construction of a multiset.

Format

<multiset value constructor> ::=
    <multiset value constructor by enumeration>
    | <multiset value constructor by query>
    | <table value constructor by query>

<multiset value constructor by enumeration> ::=
    MULTISET <left bracket or trigraph> <multiset element list> <right bracket or trigraph>

<multiset element list> ::=
    <multiset element> [ { <comma> <multiset element> }... ]

<multiset element> ::=
    <value expression>

<multiset value constructor by query> ::=
    MULTISET <table subquery>

<table value constructor by query> ::=
    TABLE <table subquery>