mysql runtime error

Question

select count(*), concat(database(), 0x3a, floor(rand(0) * 2)) as y from information_schema.tables group by y
The above query is mostly used in mysql error-based sql injection to trigger runtime error. I don't want to know about sql injection. But what I want to know is why the runtime error is caused whenever the above query is run.

Answer 1

the second part of the select statement is randomly generated count(0) will return the count of rows found but no column will match the second select