sparql - Convert String Variable to URI using SPARQL

Question

Using a string as an URI works using:

SELECT DISTINCT ?db_uri ?uri ?db_uri_var ?prop ?val

{
BIND(URI("http://dbpedia.org/resource/Delta_Air_Lines") as ?db_uri)
      
      SERVICE <http://DBpedia.org/sparql>
{
?db_uri ?prop ?val .
}
}

However, as the value for the string comes "from somewhere else", I would need to use a variable inside the URI-call. This seems not to work.

SELECT DISTINCT ?db_uri ?uri ?db_uri_var ?prop ?val

{
BIND("http://dbpedia.org/resource/Delta_Air_Lines" as ?uri)
BIND(URI(?uri) as ?db_uri)
      
      SERVICE <http://DBpedia.org/sparql>
{
?db_uri ?prop ?val .
}
}