lexer: support more syntaxes regarding 'SET Syntax'

[qazbnm456]

What have you changed? (mandatory)

I've changed lines of the code of lexer.go in order to support more syntaxes regarding 'SET Syntax', which also fixes #6990.

What are the type of the changes (mandatory)?

Improvement.

How has this PR been tested (mandatory)?

Manual test.

Does this PR affect documentation (docs/docs-cn) update? (optional)

No.

Refer to a related PR or issue link (optional)

#6990

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[shenli]

@qazbnm456 Thanks!

[shenli]

/ok-to-test

[shenli]

/ok-to-test

[shenli]

/run-all-tests

[tiancaiamao]

Is there a MySQL document link about it? @qazbnm456

[qazbnm456]

@tiancaiamao Yup, here is the docs: https://dev.mysql.com/doc/refman/5.7/en/user-variables.html.

[shenli]

/run-all-tests

[jackysp]

@qazbnm456 thank you for your contribution! Please add some test cases in

tidb/parser/parser_test.go

Line 484 in 10151e1

func (s *testParserSuite) TestDBAStmt(c *C) {

[qazbnm456]

@JackDrogon Added! Thanks.

[tiancaiamao]

Are they all equal?
@
@''
@""

@``

How about this:

set @ = 3;
select @'';

@qazbnm456

[qazbnm456]

@tiancaiamao

Yes, they're the same in MySQL. Example: http://sqlfiddle.com/#!9/9eecb/46076.
After applying this patch to the master branch of TiDB, it works the same:

[tiancaiamao]

I mean, could this PR handle the case, in TiDB

[qazbnm456]

@tiancaiamao I just updated my comment with a picture showing that the case would be handled correctly in TiDB.

[tiancaiamao]

LGTM

[jackysp]

LGTM

[jackysp]

/run-all-tests