Question

In my database, many tables have the ‘State’ field, representing the state that, that particular entity falls in. I have been told that we should use Lookup tables for this kind of thing, but I am unsure of the exact mechanism. Can someone clarify these points?

1. How is the integrity maintained? (i.e. how do I make sure that only the values from the state table go into the other tables?)

2. Does the state name go into the other tables, or does the state id from the state table go into the other tables?

Answer 1

1 – Integrity is maintained using what is called a FOREIGN KEY constraint. A reasonable scenario might have you do these two tables:

Table Name: STATE_CODE
ID    DESCRIPTION
=================
1   Alabama
2   Arkansas
...
50  Wyoming

Table Name: CUSTOMER
=====================
CUST_ID   CUST_NAME   CUST_STATE
100       AAA Company          1  --they are in Alabama!
200       ZZZ Company          50 --they are in Wyoming!

This answers your question #2: The state codes, not the full names, go in the CUSTOMER table in this example.

A typical script to impose this kind of structure on an existing layout would be like this:

--first, create the lookup table
CREATE TABLE STATE_CODE(
  ID INTEGER NOT NULL
 ,DESCRIPTION VARCHAR(100) NOT NULL
 ,PRIMARY KEY(ID)
);

--now add a reference to the lookup table inside your existing table
--the REFERENCES part will **force** entries
--to have a matching entry in STATE_CODE
ALTER TABLE CUSTOMER ADD STATE_CODE_ID REFERENCES STATE_CODE(ID);

And this answers your question #1: That “REFERENCES” command will create a Foreign Key constraint that will force all entries in CUSTOMER.STATE_CODE to have a corresponding entry in the STATE_CODE table. After setting this up, if someone were to try this:

INSERT INTO CUSTOMER(CUST_ID,CUST_NAME,CUST_STATE)
VALUES(9000,'Martians',74837483748);

Then they would get an error message, and that faulty data would never get entered (unless, of course, you really did have a state with a code of 74837483748).