We now have a starting point for a physical schema. We have not yet translated the relationships into the physical data model. As we discussed earlier, once you have refined your data model, you should have all 1-to-1 and 1-to-M relationships—the M-to-M relationships were resolved via junction tables. We model relationships by adding a foreign key to one of the tables involved in the relationship. A foreign key is the unique identifier, or primary key, of the table on the other side of the relationship.

The most common relationship is the 1-to-M relationship. This relationship is mapped by placing the primary key from the “one” side of the relationship into the table on the “many” side. In our example, this rule means we need to do the following:

Table 7-3 shows the new schema.

We do not have any 1-to-1 relationships in this data model. If we did have such a relationship, we would map it by picking one of the tables and giving it a foreign key column that matches the primary key from the other table. In theory, it does not matter which table you choose, but practical considerations may dictate which column makes the most sense as a foreign key. Another way to handle a 1-to-1 relationship is to simply combine both entities into a single table. In that case, you have to pick a primary key from one of the tables to be the primary key of the combined table.

We now have a complete physical database schema. The last remaining task is to translate that schema into SQL. For each table in the schema, you write one CREATE TABLE statement. Typically, you should create unique indexes on the primary keys to enforce uniqueness.

Example 7-1 is an example SQL script for creating the example database in MySQL.

CREATE TABLE cd (cd_id            INT NOT NULL PRIMARY KEY,
                 record_label     INT,
                 cd_title         VARCHAR(50));

CREATE TABLE artist (artist_id    INT NOT NULL PRIMARY KEY,
                     artist_name  VARCHAR(50));

CREATE TABLE song (song_id        INT NOT NULL PRIMARY KEY,
                   song_name      VARCHAR(50),
                   song_length    TIME,
                   cd_id          INT,
                   artist_id      INT);

CREATE TABLE record_label (record_label_id    INT NOT NULL PRIMARY KEY,
                           record_label_name  VARCHAR(50));

Note that no FOREIGN KEY reference is used in the script. This is because MySQL does not support FOREIGN KEY constraints in its default data type. MySQL will allow you to embed them in your CREATE TABLE statements, but they will not be enforced. The InnoDB table type, which was recently stabilized and is documented on the MySQL web site, supports foreign keys.

Data models are meant to be database independent. You can therefore take the techniques and the data model we have generated in this chapter and apply them not only to MySQL, but to Oracle, Sybase, or any other relational database engine. In the following chapters, we will discuss the details of how you can use your new database design knowledge to build applications.