c++ expects ';' at end of member declaration when there is one - c++

Question

I have two errors that arise on the same line of code.
The following are snippets, only relevant code is there.
First is Database Class
#include "program.h"
using namespace std;
class Database {
public:
Database(Program *program);
~Database();
};
Next is the Query Class.
#include "database.h"
using namespace std;
class Query {
public:
Relation* evaluate(Database* database);
};
The line in question is:
Relation* evaluate(Database* database);
The first error is "Database does not name a type", despite including the database.h file.
The second error is "expected ';' after member declaration" which I believe is related to the first.
Everything in this project has worked until the addition of this database class.
I am stumped on this one, and have been researching all over stackoverflow to no avail.
I am on Debian Stretch and compiling with g++. Does anyone have any ideas?
EDIT
Minimal compilable code:
query.h
#include "database.h"
class Query {
public:
void evaluate(Database* database);
};
program.h
#include "query.h"
#include "database.h"
using namespace std;
class Program {
public:
};
database.h
#include "program.h"
class Database {
public:
Database(Program *program);
};

Answer 1

You might want to try out the -E option of the compiler: it causes the source code to be preprocessed, i.e., you'll see the code as it is seen by the compiler. If you look at your headers closely you'll see that there are cyclic dependencies between the different types. You'll need to break them.
The "easy" fix is to declare the Database class in query.h:
class Database;
class Query {
public:
void evaluate(Database* database);
};
While this will fix the shown problem, cyclic dependencies will come and haunt you. In general they are bad. A system of class which cyclicly depend on each other are effectively just one component and should probably be all declared in just one header.