Remove diacritical marks in SQL

Hello,

I would like to remove diacritical marks from a string in a SQL query.
I tried to convert a UTF8 string to ASCII but it doesn't work for me.

SELECT convert('ěščřžýáíé','UTF8','SQL_ASCII')

array(1) {
  ["convert"]=>
  string(18) "ěščřžýáíé"
}

Thanks for any advice,

J.N.