This KATA looks like cannot be solved by SQL at all because of this case "when no letter at all".

When I simply run the code "select * from letters", it gave me the below errors, which means it cannot be ran in SQL environment at all? Is it a bug?

undefined method `[]' for nil:NilClass
Simplified backtrace: block in run_test

Hi I have passed 56 test cases, but not sure there are 7 tests which always cannot pass.

It gave me below errors:

Test Failed
@@ ,when_happened ,visits_count
+++,2019-12-14 08:57:36 +0000,17
---,2019-12-11 08:57:36 +0000,33

or

Test Failed
@@,when_happened ,visits_count
→ ,2019-04-01 01:00:00 +0000,3→2

I am not able to see the full test cases so wondering what are the edge cases which blocked my passing?

I can run my code in local SQL client very well but in codewar, it always told me "undefined method `[]' for nil:NilClass Simplified backtrace: block (4 levels) in spec". Really don't understand what is the issue.

Me too. And I cannot use "With ... AS ()" subquery to escape the single quote, it will tell me the table not exists.