Question

I’m a newbie to mysql. I knew this is a very basic question, but I could not figure out how 🙁 ?
My initial attempt:

CREATE TABLE Node(  
  Id INT( 3 ),  
  Position VARCHAR( 12 ),  
  FoodTax INT( 2 ),  
  HasTreasureMap TINYINT( 1 ),  
  CurrentPlayer Player, 
  PRIMARY KEY( Id ) 
);

In this case, ‘Player’ is another table that I want to add. How can I do this?

Answer 1

You can not add Tables inside tables, what you do is add a foreign key reference, which is basically the ID (or whatever the PRIMARY KEY is) of the other table and make the other table separately.

CREATE TABLE Node (  
  Id INT( 3 ),  
  Position VARCHAR( 12 ),  
  FoodTax INT( 2 ),  
  HasTreasureMap TINYINT( 1 ),  
  CurrentPlayer INT( 3 ),
  PRIMARY KEY( Id ),
  INDEX ( CurrentPlayer )
);

CREATE TABLE Player (
  Id INT( 3 ),
  Name VARCHAR( 64 ),
  PRIMARY KEY ( Id )
);

Note that INDEX( CurrentPlayer ) in the first table is not necessary unless you are going to join the tables, or use the CurrentPlayer field to search or sort on.

If you are using the InnoDB database engine, then you can directly specify the nature of the relationship between the tables within the definition for the Node table as follows.

CREATE TABLE Node (  
  Id INT( 3 ),  
  Position VARCHAR( 12 ),  
  FoodTax INT( 2 ),  
  HasTreasureMap TINYINT( 1 ),  
  CurrentPlayer INT( 3 ),
  PRIMARY KEY( Id ),
  INDEX ( CurrentPlayer ),
  FOREIGN KEY ( CurrentPlayer ) REFERENCES Player( Id )
    ON UPDATE CASCADE
    ON DELETE SET NULL
);

The FOREIGN KEY addition causes a few effects, it adds a check in so that you can not set a value for CurrentPlayer in the Node table that does not already exist in the Player table, any INSERT/UPDATE attempting this would fail. Also, the two options ON UPDATE and ON DELETE tell the database that if a record from Player is updated or deleted, then perform an action automatically via a trigger on the Node table. ON UPDATE CASCADE means that if the Id of a Player record was updated to something else, it would cascade in to the Node table, changing that value too. ON DELETE SET NULL states that if a record is deleted from the Player table, then corresponding CurrentPlayer values in Node should be set to NULL.