Defining constant values in struct

[tcak via Digitalmars-d-learn]

As far as I known, when I define a string with enum and it is 
used at different parts of code, that string is repeated again 
and again in executable file instead of passing a pointer to 
string. So, using enum with string doesn't seem like a good idea.

Hence, I defined string as const to make it belong to struct 
itself instead of instances, but it comes with 'need `this` for 
...' error. This indicates that the string doesn't belong to 
struct itself actually.

Because there is nothing like namespace in D, I used a sub-struct.

[code]
struct TableSchema{
	const string TABLE = "users";

	struct FieldTypes{
		const string ID = "BIGINT";
	}

	const string CREATESQL = "... id " ~ FieldTypes.ID ~ "...";
}
[/code]


But compiler doesn't allow me to access FieldTypes.ID. It says 
that it needs `this`. I tried with `static shared`, used `class` 
instead of `struct` etc. But couldn't have come up with a nice 
solution.