The correct syntax for using with global keyword. To access a global variable in WordPress, you first need to globalize the variable with global $variable;
Write inside function.php as:
function myfunction(){
global $get_variable;
$get_value = $db->query("SELECT * FROM mytable")->fetch();
$get_variable = $get_value;
}
add_action( 'after_theme_setup', 'myfunction' );
Inside the function you can now able to any where. But, outside of this scope it needs to be re-declared as a global scope variable. Ex., to use inside single.php file, it will work as
global $get_variable;
echo $get_variable;