Weird behaviour of SUM and CONCAT in MySql

Question

If I want to make a sum of a specific numeric column in MySQL, I do

SELECT SUM(MyColumn) FROM MyTable WHERE 1;

This returns for example num

Answer 1

As FreshPrinceOfSO suggested in the comments below my question, MySQL server doesn't handle casts to varchar.

So even though the query

SELECT CONCAT('Sum is: ',CAST(SUM(MyColumn) AS varchar(20))) FROM MyTable WHERE 1;

results in syntax error, casting to char instead works just fine:

SELECT CONCAT('Sum is: ',CAST(SUM(MyColumn) AS char(20))) FROM MyTable WHERE 1;