Add CREATE TABLE AS support

[Dandandan]

Issue: https://github.com/andygrove/sqlparser-rs/issues/205

[coveralls]

Pull Request Test Coverage Report for Build 144885262

• 49 of 51 (96.08%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 91.837%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	13	14	92.86%
tests/sqlparser_postgres.rs	14	15	93.33%

Files with Coverage Reduction	New Missed Lines	%
src/ast/mod.rs	1	81.17%

Totals
Change from base Build 137652853:	0.03%
Covered Lines:	4174
Relevant Lines:	4545

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💛 - Coveralls

[nickolay]

Thanks! I've added my thoughts inline.. Of those I'd like query.is_none() to be fixed before merging, for the rest I'm curious what you think.

[nickolay]

I just realized that CREATE TABLE t (a INT) AS SELECT a from t2 is BigQuery-specific, while in both PostgreSQL and ANSI SQL a plain list of columns (without datatypes) in expected in the parentheses after the table name in a CTAS statement.

I also wanted to have a test for that and for CREATE TABLE t (CONSTRAINT positive CHECK (2 > 1)) you mentioned above, so I added a commit with those.

I think this is ready to be merged, what do you think?

[Dandandan]

I think so too! Thanks a lot :)

[nickolay]

Excellent, thanks! I summarized the changes here in the commit message as follows:

We parse it as a regular CREATE TABLE statement followed by an AS <query>, which is how BigQuery works: https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#create_table_statement

ANSI SQL and PostgreSQL only support a plain list of columns
after the table name in a CTAS CREATE TABLE t (a) AS SELECT a FROM foo
We currently only allow specifying a full schema with data types, or omitting it altogether.
https://www.postgresql.org/docs/12/sql-createtableas.html https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#as-subquery-clause

Finally, when no schema is specified, we print empty parens after a plain CREATE TABLE t (); as required by PostgreSQL, but skip them in a CTAS: CREATE TABLE t AS .... This affects serialization only, the parser allows omitting the schema in a regular CREATE TABLE too since the first release of the parser: https://github.com/benesch/sqlparser-rs/blame/7d27abdfb4ac4a0980d79b12d9d8ef156434e984/src/sqlparser.rs#L325-L332