Support parsing create external table.

[zhzy0077]

Add the possibility to parse the following SQL:

CREATE EXTERNAL TABLE uk_cities (
         name VARCHAR(100) NOT NULL,
         lat DOUBLE NULL,
         lng DOUBLE NULL
)
STORED AS TEXTFILE
LOCATION '/tmp/example.csv'

[coveralls]

Pull Request Test Coverage Report for Build 142

• 121 of 143 (84.62%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 85.225%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_generic.rs	34	35	97.14%
src/sqlparser.rs	53	60	88.33%
src/sqlast/mod.rs	25	39	64.1%

Files with Coverage Reduction	New Missed Lines	%
src/sqlparser.rs	1	80.59%

Totals
Change from base Build 139:	0.1%
Covered Lines:	2613
Relevant Lines:	3066

---

💛 - Coveralls

[andygrove]

LGTM!

[andygrove]

@nickolay Any opinion on this PR?

[nickolay]

Thanks!

LGTM as well, though I'd remove the second (parse_create_external_table_newline) test, since it appears to differ only in whitespace (newline), which is stripped by the tokenizer anyway.

I don't think creating a separate SQLStatement variant for EXTERNAL tables specifically is right, since most of the syntax is shared by CREATE TABLE and CREATE EXTERNAL TABLE (I believe the only difference is that the LOCATION parameter is required for EXTERNAL tables and can't be specified for managed ones). I don't know how the dialect-specific DDL would be best represented though, so a separate variant, as in this PR, looks as good a starting point as any.

Syntax:

• https://cwiki.apache.org/confluence/display/Hive/LanguageManual+DDL#LanguageManualDDL-CreateTable
• https://www.cloudera.com/documentation/enterprise/6/6.2/topics/impala_create_table.html

[zhzy0077]

@nickolay I think it's more intuitive if we have inheritance here. CreateExternalTable will be a subclass of CreateTable and share most fields. However I am a newbie to Rust and don't know how to represent it in rustlang.

[nickolay]

Rust is “composition over inheritance”, so what you’re thinking about is not an option. The only alternative way to add EXTERNAL table parsing without way more work is to add Options for the location and the file format in the existing SQLCreateTable variant. Either way is fine with me.

[andygrove]

Thanks @nickolay for the review.

For dialect, I am keen on using Hive syntax. I would like to see official support for HiveQL in this crate eventually.

[andygrove]

@zhzy0077 I agree that it makes sense to combine SQLCreateTable and SQLCreateExternalTable and use Option for attributes that are only in the external variant, and add an attribute to indicate whether this is EXTERNAL or not. Just a bool would be fine for now.

[zhzy0077]

@andygrove I have updated this pr according to your and Nickolay's advice. Thanks for the comprehensive explanation! @nickolay