CASE with alias is not properly parsed

Feeding this SQL to the Parser:

SELECT
  CASE 
    WHEN a.kol = 1 THEN 'one'
    ELSE a.kol
  END as 'case'
FROM foo a;

will result in errors, and calling build() on the statement will result in this SQL:

SELECT CASE 'case' ELSE a.kol AS `END` END FROM foo AS `a`

The errors array contains these errors:

• An alias was previously found.
• An alias was expected.
• Unexpected keyword.
• Unexpected end of CASE expression

I encountered this problem since I upgraded from 5.8.2 to 5.9.1.

首先通过 Parser 重现所提供的 CASE 表达式，并比较报告的错误和 build() 的输出。跟踪 CASE 表达式如何使用别名和关键字，然后添加一个回归用例，表明此 SQL 可以无错误地解析，并且 build() 生成的结果正确保留了别名。