Question

我在MySQL和

中有一个名为payment_request的表

DESCRIBE payment_request提供以下输出

下面提供了oderbook表，

我想将id中payment_request表中的orderbook添加为payment_request_id列之后名为id的外键（第二位）。

运行MySQL的SQL是什么？

Answer 1

首先，您需要在表orderbook

中添加新列

ALTER TABLE orderbook
ADD payment_request_id INT(10) unsigned AFTER ID;

然后添加一个将定义外键

的约束

ALTER TABLE orderbook
ADD CONSTRAINT fk_orderbook FOREIGN KEY (payment_request_id) 
REFERENCES payment_request (id);

参考：

MySQL Alter Table

Answer 2

您可以在创建表格时执行此操作：

 #include <iostream>
 using namespace std;
 // This function is to create the requested magic squares
int main()
{

  int n;
  //asking for n
  cout << "Please enter an odd number" << endl;
  cin >> n;

  //checking in case n doesnt follow rules
  if (n < 3 || n % 2 == 0)
  {
     cout << "Invalid Entry, Please re-enter an odd number that is 3 or larger " << endl;
  }
  else
  {
     // A function to generate odd sized magic squares

     int magicSquare[n][n];

     // Setting every slot to 0
     for (int i = 0; i < n; i++)
     {
        for (int j = 0; j < n; j++)
        {
           magicSquare[j][i] = 0;
        }
     }

     // Initializing position to 1
     int j = n / 2;
     int i = n - 1;

     // Setting each value to generate the magic square
     for (int num = 1; num <= n * n; )
     {
        if (j == -1 && i == n)
        {
            i = n - 2;
            j = 0;
        }
        else
        {
            //send to next number
            // moving it to the right side
            if (i == n)
                i = 0;
            //send to next number again
            // moving it to the upper side
            if (j < 0)
                j = n - 1;
        }
        //second condition
        if (magicSquare[j][i])
        {
            i -= 2;
            j++;
            continue;
        }
        else
            //add the number
            magicSquare[j][i] = num++;
        //first condition
        i++; j--;
    }
    //displaying sum of col/row
    cout << "The sum of each row/col: " << n * (n*n + 1) / 2 << endl;
    //Dispplaying magic square
    for (j = 0; j<n; j++)
    {
        for (i = 0; i<n; i++)
            cout << " " << magicSquare[i][j];
        cout << "\n";
    }

 }
   cout << endl;
   //re running program
   return main();
}

或通过改变表格：

CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);

另请参阅this tutorial。

如何在MySQL表中添加外键列？

2 个答案: