Matrix-like PKs based on three optional IN-constrained strings?

Question

Hello,
I have a DB of actions that I want to represent. I want to model a number of actions as you know them from classic GUIs:
open
openFile
saveAs
exportToDirectory
reverseEngineerFromDatabase
As I analyzed the problem, I came to the conclusion, the keys above (which they ultimately are) have at least three parts:
1. an action: VARCHAR(20) CHECK (action IN ('open', 'save', 'export', 'reverseEngineer', ...))
2. a preposition: VARCHAR(4) CHECK (preposition IN ('as', 'to', 'from', ...))
3. a target: VARCHAR(20) CHECK (target IN ('file', 'directory', 'database', ...))
So the rule to construct the final key would be:
<action> + <preposition> + <target>
to model all sorts of combinations/permutations (forget about uppercasing the preposition and target if there for a moment).
As you can see from the strings, I have three dimensions of predefined values from which I construct the final key. Not every combination is possible, I want to represent valid combinations by an extra table having entries like:
INSERT INTO ValidActions ('open', '', '')
INSERT INTO ValidActions ('open', '', 'file')
INSERT INTO ValidActions ('save', 'as')
INSERT INTO ValidActions ('export', 'to', 'directory')
INSERT INTO ValidActions ('reverseEngineer', 'from', 'database')
My question is now:
How do I solve this, given that the PK is composed of ... PRIMARY KEY (action, preposition, target) ... and that an action has an OPTIONAL preposition and target? I have not come to a final decision, but I might relax the action to be optional, too, and then demand that either <action> OR <target> MUST be set.
How do I model matrix-like composed PKs consisting of a number of optionals? I currently see no other way of adding empty strings '' to the check constraint, however AFAIK Oracle does not make a difference between NULL and the empty string ''. How would such a solution look like?
I could use the final string as PK, but how do I make sure that string consists of the predefined ones only?
Karsten
PS: note action and target are actually two separate tables.

Answer 1

What about using a model like this? I used surrogate keys because I wasn't sure if the ACTION, PREPOSITION and TARGET values were truly immutable. You could easily remove these from the model and use the respective columns as the PK for each table.
DROP TABLE ACT_PRE_TAR;
DROP TABLE ACTIONS;
CREATE TABLE ACTIONS
        ACTION_ID       NUMBER  PRIMARY KEY
,       ACTION          VARCHAR2(15) NOT NULL
,       CONSTRAINT ACTION#UNQ UNIQUE (ACTION)
INSERT INTO ACTIONS VALUES(1,'open');
INSERT INTO ACTIONS VALUES(2,'save');
INSERT INTO ACTIONS VALUES(3,'export');
INSERT INTO ACTIONS VALUES(4,'reverseEngineer');
DROP TABLE PREPOSITIONS;
CREATE TABLE PREPOSITIONS
        PREPOSITION_ID  NUMBER  PRIMARY KEY
,       PREPOSITION     VARCHAR2(10) NOT NULL
,       CONSTRAINT PREPOSITION#UNQ UNIQUE (PREPOSITION)
INSERT INTO PREPOSITIONS VALUES (1,'as');
INSERT INTO PREPOSITIONS VALUES (2,'to');
INSERT INTO PREPOSITIONS VALUES (3,'from');
DROP TABLE TARGETS;
CREATE TABLE TARGETS
        TARGET_ID       NUMBER  PRIMARY KEY
,       TARGET          VARCHAR2(10) NOT NULL
,       CONSTRAINT TARGETS#UNQ UNIQUE (TARGET)
INSERT INTO TARGETS VALUES (1,'file');
INSERT INTO TARGETS VALUES (2,'directory');
INSERT INTO TARGETS VALUES (3,'database');
CREATE TABLE ACT_PRE_TAR
        ACTION_ID       NUMBER  REFERENCES ACTIONS(ACTION_ID)
,       PREPOSITION_ID  NUMBER  REFERENCES PREPOSITIONS(PREPOSITION_ID)
,       TARGET_ID       NUMBER  REFERENCES TARGETS(TARGET_ID)
,       CONSTRAINT ACT_PRE_TAR#UNQ UNIQUE (ACTION_ID, PREPOSITION_ID, TARGET_ID)
);Based on your sample data the following result is produced:
SQL> SELECT  ACTION
  2  ,       PREPOSITION
  3  ,       TARGET
  4  FROM            ACT_PRE_TAR
  5  LEFT OUTER JOIN ACTIONS         ON ACTIONS.ACTION_ID            = ACT_PRE_TAR.ACTION_ID
  6  LEFT OUTER JOIN PREPOSITIONS    ON PREPOSITIONS.PREPOSITION_ID  = ACT_PRE_TAR.PREPOSITION_ID
  7  LEFT OUTER JOIN TARGETS         ON TARGETS.TARGET_ID            = ACT_PRE_TAR.TARGET_ID
  8  /
ACTION          PREPOSITIO TARGET
open
open                       file
save            as
export          to         directory
reverseEngineer from       database