You are needing to convert it to a string because you are creating an SQL query AS A STRING
(using prepared statements would be a bit different, as you numeric values could stay numeric)
And either STR(), FORMAT() or CSTR() depending on your requirements
Ideally, have a numeric field defined and as Dave says, send a number as a parameterised query
If you must turn it into a string before storing,
I advise use STR()
Why?
If you use CSTR() and the app is used in both the USA and in France, then what is written to the database will differ.
USA will get 4211750882.72
France will get 4211750882,72
(And possibly a few more commas and points along the way at the thousand mark)
When you read that back, if you try to do maths with it, it may fail to parse properly.
Using STR will always do it the American Way.
Getting the number back as a number … use VAL() which expects the American Format
Or, as Dave mentioned, use prepared statements and don’t worry about what to use when. You leave it as an number and let the database do what is right.