Extract all unique chars from all texts across the table
MySQL has no way to split a cell into a bunch of values.  (Yeah, it can split a piece of a cell off -- see LEFT(), etc.  But not an arbitrary set.)

You would be better off doing the task in PHP or some other language -- more powerful than SQL.  (Yeah, it could be done in a Stored Procedure, the the code would be really messy.)

Here is some pseudo code for PHP (etc):

declare $hash as an associative array
foreach row of SELECT * FROM tbl
    foreach cell
        split cell into characters
            $hash[$char] = 1
dump the _keys_ of $hash


(Getting the counts is a trivial enhancement.)

Be sure to work in "characters", not "bytes".  The details of that depend on the language.