Primary key column does not allow null values

Question

I have a question concerning primary key or compound key. I have created a table where there are only two attributes and both are are primary keys.

The primary keys name are item_ID̀ andSName`

These two are foreign keys to another primary key (to two other tables) where: item_ID --> ID and SName --> SkillName

ITEM ID      SName

1        Basic weapons
2   
3   
4        Heavy weapon
5        Exotic Weapons
6        Heavy weapon
7        Power weapon

and the outcome is this:

The ID nr 2 and 3 does not need a skillname requirement. For that reason I have implemented like this (since a primary key is not allowed a null):

INSERT INTO Requierments (item_ID, SName) VALUES ( 
01, 'Basic weapons');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
02, null);

INSERT INTO Requierments (item_ID, SName) VALUES ( 
03, null);

INSERT INTO Requierments (item_ID, SName) VALUES ( 
04, 'Heavy weapon');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
05, 'Exotic Weapons');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
06, 'Heavy weapon');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
07, 'Power weapon');

Is that correctly done? or is there another way to code that the ID does not require a specific SName?

---

here is the code for the 3 tables:

CREATE TABLE `talents` (
  `SkillName` varchar(30) NOT NULL DEFAULT '',
  `Bonus` varchar(30) DEFAULT NULL,
  `Description` varchar(70) DEFAULT NULL,
  `R_Str` int(11) DEFAULT NULL,
  `R_WS` int(11) DEFAULT NULL,
  `R_BS` int(11) DEFAULT NULL,
  `R_Fel` int(11) DEFAULT NULL,
  `R_Per` int(11) DEFAULT NULL,
  `R_Int` int(11) DEFAULT NULL,
  `R_Agi` int(11) DEFAULT NULL,
  `R_WP` int(11) DEFAULT NULL,
  `Talent_requiert` varchar(30) DEFAULT NULL,
  PRIMARY KEY (`SkillName`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `requierments` (
  `item_ID` int(11) NOT NULL DEFAULT '0',
  `SName` varchar(30) NOT NULL DEFAULT '',
  PRIMARY KEY (`item_ID`,`SName`),
  CONSTRAINT `requierments_ibfk_1` FOREIGN KEY (`item_ID`) REFERENCES `item` (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE `item` (
  `ID` int(11) NOT NULL DEFAULT '0',
  `Name_` varchar(30) DEFAULT NULL,
  `Weight` int(11) DEFAULT NULL,
  `Value_` int(11) DEFAULT NULL,
  `Availability` varchar(30) DEFAULT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

The idea is in table requirements item_ID ---> ID in item table and SName ---> SkillName in talents. for some reason i was able to make a foreign key from item_ID to ID but cant from SName to SkillName:

the values i have added to those tables are the followings (OBS that in the table requirements i have set ID 2 and 3 to '' since the dont need any talent to be able to select it. but i am unsure if i am allowed to use that method?

INSERT INTO Talents ( SkillName, Bonus, Description, R_Str, R_WS, R_BS, R_Fel, R_Per, R_Int, R_Agi, R_WP, Talent_requiert) VALUES (
 'ambidextrous', 0, 'use either hand equally well', 0, 0, 0, 0, 0, 0, 30, 0, null);

INSERT INTO Talents ( SkillName, Bonus, Description, R_Str, R_WS, R_BS, R_Fel, R_Per, R_Int, R_Agi, R_WP, Talent_requiert) VALUES (
 'Frenzy', 0, 'enter psychotic rage to gain combat bonus', 0, 0, 0, 0, 0, 0, 0, 0, null);

INSERT INTO Talents ( SkillName, Bonus, Description, R_Str, R_WS, R_BS, R_Fel, R_Per, R_Int, R_Agi, R_WP, Talent_requiert) VALUES (
 'battle rage', 0, 'parry while frenzied', 0, 0, 0, 0, 0, 0, 0, 0, 'Frenzy');

INSERT INTO Talents ( SkillName, Bonus, Description, R_Str, R_WS, R_BS, R_Fel, R_Per, R_Int, R_Agi, R_WP, Talent_requiert) VALUES (
 'Exotic Weapons', 0, 'Player is able to use exotic weapons', 0, 0, 0, 0, 0, 0, 0, 0, 'Basic weapons');

INSERT INTO Talents ( SkillName, Bonus, Description, R_Str, R_WS, R_BS, R_Fel, R_Per, R_Int, R_Agi, R_WP, Talent_requiert) VALUES (
 'Basic weapons', 0, 'Player is able to use Basic weapons', 0, 0, 0, 0, 0, 0, 0, 0, null);

INSERT INTO Talents ( SkillName, Bonus, Description, R_Str, R_WS, R_BS, R_Fel, R_Per, R_Int, R_Agi, R_WP, Talent_requiert) VALUES (
 'Heavy weapon', 0, 'Player is able to use heavy weapons', 30, 0, 0, 0, 0, 0, 0, 0, null);

INSERT INTO Talents ( SkillName, Bonus, Description, R_Str, R_WS, R_BS, R_Fel, R_Per, R_Int, R_Agi, R_WP, Talent_requiert) VALUES (
 'Power weapon', 0, 'Player is able to use power weapons ', 40, 30, 0, 0, 0, 0, 0, 0, null);



INSERT INTO Item ( ID, Name_, Weight, Value_, Availability) VALUES (
01, 'Las Carbine', 3, 75, 'Common' );

INSERT INTO Item ( ID, Name_, Weight, Value_, Availability) VALUES (
02, 'Laspistol', 1, 50, 'Common' );

INSERT INTO Item ( ID, Name_, Weight, Value_, Availability) VALUES (
03, 'Shotgun', 5, 60, 'average' );

INSERT INTO Item ( ID, Name_, Weight, Value_, Availability) VALUES (
04, 'Heavy Bolter', 40, 2000, 'Very Rare' );

INSERT INTO Item ( ID, Name_, Weight, Value_, Availability) VALUES (
05, 'Needle pistol', 2, 1250, 'Very Rare' );

INSERT INTO Item ( ID, Name_, Weight, Value_, Availability) VALUES (
06, 'Chainsword', 6, 275, 'Rare' );

INSERT INTO Item ( ID, Name_, Weight, Value_, Availability) VALUES (
07, 'Power Sword', 4, 2500, 'Very Rare' );


INSERT INTO Requierments (item_ID, SName) VALUES ( 
01, 'Basic weapons');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
02, '');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
03, '');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
04, 'Heavy weapon');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
05, 'Exotic Weapons');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
06, 'Heavy weapon');

INSERT INTO Requierments (item_ID, SName) VALUES ( 
07, 'Power weapon');

Answer 1

You are trying to deal with the situation where an Item has no Requirement. In other words, such an Item has no relation with a Talent: simply do not insert anything in Requirements for this Item!