Support for DuckDB Union datatype

[gstvg]

e.g. CREATE TABLE tbl1 (u UNION(num INTEGER, str VARCHAR));

https://duckdb.org/docs/sql/data_types/union.html

[coveralls]

Pull Request Test Coverage Report for Build 9651403373

Details

• 93 of 94 (98.94%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 89.039%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser/mod.rs	15	16	93.75%

Totals
Change from base Build 9633122504:	0.03%
Covered Lines:	26369
Relevant Lines:	29615

---

💛 - Coveralls

[gstvg]

Zero field unions are invalid on DuckDB and don't even parse. Should we error here using parse_comma_separated or let downstream users handle it?

https://shell.duckdb.org/#queries=v0,create-table-tbl1-(u-union()-)~

[jayzhan211]

I think it is fine to return error here.

[gstvg]

Done

[jayzhan211]

👍

[coveralls]

Pull Request Test Coverage Report for Build 9664186307

Details

• 88 of 89 (98.88%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 89.037%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser/mod.rs	15	16	93.75%

Totals
Change from base Build 9633122504:	0.03%
Covered Lines:	26364
Relevant Lines:	29610

---

💛 - Coveralls

[iffyio]

LGTM! cc @alamb

[alamb]

Likewise -- looks good to me -- thank you @iffyio and @gstvg