Question: I have 2 db tables - one is including all the entries, the other one is the "blacklist" - means i want to select all values excluding the entries of table 2

```
$req = "SELECT * FROM table1 WHERE link_id != (SELECT id FROM table2)";
$stmt = $pdo->query($req);

foreach($stmt as $data) {

echo $data['link_id']."<br>";           

}

```

Can anyone help me out?

Answer: 
You say that

> 
> I've also tried the stdbool.h library , however it didn't work because the library wasn't found.
> 
> 
> 

I'm inclined to suggest, then, that you find and use a conforming C99 or C2011 compiler. The former, at least, should not be too hard to put your hands on. Either will assuredly provide the header, and using it is probably your most convenient way forward.

Inasmuch as your code still contains some C++-isms (e.g. `cout`, a `using` statement, and C++-style `#include`), I'm inclined to believe that you are compiling with a C++ compiler. That is one conceivable reason why the `stdbool.h` header is not found. If you want to convert to C, then be sure to build your code with a C compiler.
C99 has the `<stdbool.h>` header.