String handling in T sql for mysql

Question

I have a variable

DECLARE url varchar(3000)

and incoming parameters

scheme VARCHAR(5),
subdomain VARCHAR(55),
domain VARCHAR(55),
tld VARCHAR(55)

yet when i do the following:

SET url = url + ltrim(rtrim(scheme)) + '://';
SET url = url + ltrim(rtrim(subdomain)) + '.';
SET url = url + ltrim(rtrim(domain));
SET url = url+ '.' + ltrim(rtrim(tld));

And insert to the database i get "0"

I'm a newb with sql, what am i doing wrong?