#include<iostream>
// try to distinguish the difference between the following setences
extern"C" int x; //declaration
extern"C" {int y;} //definition
int main()
{
int x;
std::cout<<x<<y;
return 0 ;
}

array in java
int a[ ] = new int a[MAX]； // reference type , not value type , so "b = a " does not mean copy by value
continue in java
here:
for(……) {
for(......) {
continue here;
}
}
for each in java
for( int k:array) { //read only
}
for each in C#
foreach(int k in array) { //read only
}

class A
{
private:
int i;
public:
A(int x = 1):i(x) { }
void print() { cout<<i<<endl; }
}
int main()
{
A a;
int *p = &a;
*p = 16;
a.print();
}
we know that the private in C++ is not always true.

natural join
unlike normal join, it's not all the possible compositions of the tuples, it only contains the mutual tuples in the common attributes.
When we connect two tables, what we care about is : which attribute we choose to base on, and which tuple we choose as the results.
For the former one, we have join and natural join, and for the latter ,we have inner join, left outer join, right outer join and full outer join.

a natural join s on (condition) using (attributes)

It's easy to understand natural(outer/inner)join
Inner join with the equality judgement : it does not discard the duplication.
In outer join, other attributes are null or default
But , normal join with using, discard the duplication, it's similar to natural join.