Support EXISTS subqueries

[benesch]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 238

• 26 of 29 (89.66%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.877%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_common.rs	17	20	85.0%

Totals
Change from base Build 234:	0.0%
Covered Lines:	1465
Relevant Lines:	1630

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 258

• 30 of 32 (93.75%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 90.579%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_common.rs	22	24	91.67%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	4	91.43%

Totals
Change from base Build 256:	0.01%
Covered Lines:	3394
Relevant Lines:	3747

---

💛 - Coveralls

[nickolay]

LGTM!

I'm assuming NOT EXISTS() will parse correctly with unary NOT, unsure if it deserves a test of its own.

This fixes the last remaining part of #5, thanks!

[benesch]

I'm assuming NOT EXISTS() will parse correctly with unary NOT, unsure if it deserves a test of its own.

Yep! I added a test just to be sure.

This fixes the last remaining part of #5, thanks!

Oh, nice. I've updated the commit message to close out #5 when this merges.