Support for (+) outer join syntax

[jmhain]

Snowflake supports a peculiar alternative syntax for outer joins using the (+) operator. For example, the following two queries are logically equivalent:

SELECT t1.c1, t2.c2
FROM t1, t2
WHERE t1.c1 = t2.c2 (+);

SELECT t1.c1, t2.c2
FROM t1 LEFT OUTER JOIN t2
        ON t1.c1 = t2.c2;

This adds support to the parser for this syntax. I've only tested on Snowflake, but a web search suggested that MS SQL also supports this syntax, so I permitted it on that dialect as well.

[coveralls]

Pull Request Test Coverage Report for Build 8011843286

Details

• 0 of 49 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 87.713%

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Totals
Change from base Build 7915258585:	0.03%
Covered Lines:	19702
Relevant Lines:	22462

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💛 - Coveralls

[alamb]

Snowflake supports a peculiar alternative syntax for outer joins using the (+) operator. For example, the following two queries are logically equivalent:

I recognize that syntax from Oracle as well https://blogs.oracle.com/optimizer/post/outerjoins-in-oracle 👍

[alamb]

Looks great to me -- thank you very much @jmhain . The comments and tests are 💯