Creating primary and foreign keys

The CREATE TABLE and ALTER TABLE statements allow many attributes of tables to be set, including column constraints and checks. This section shows how to set table attributes using the primary and foreign keys as an example.

The following statement creates the same skill table as before, except that a primary key is added:

CREATE TABLE skill (
  skill_id INTEGER NOT NULL,
  skill_name CHAR( 20 ) NOT NULL,
  skill_type CHAR( 20 ) NOT NULL,
  primary key( skill_id )
)

The primary key values must be unique for each row in the table which, in this case, means that you cannot have more than one row with a given skill_id. Each row in a table is uniquely identified by its primary key.

Columns in the primary key are not allowed to contain NULL. You must specify NOT NULL on the column in the primary key.

Each foreign key relationship relates a candidate key (primary key and unique constraint) in one column to a column in another table, which becomes the foreign key.

For example, you can create a table named emp_skill, which holds a description of each employee's skill level for each skill in which they are qualified, as follows:

CREATE TABLE emp_skill(
emp_id INTEGER NOT NULL,
skill_id INTEGER NOT NULL,
"skill level" INTEGER NOT NULL,
PRIMARY KEY( emp_id, skill_id ),
FOREIGN KEY REFERENCES employee,
FOREIGN KEY REFERENCES skill
)

The emp_skill table definition has a primary key that consists of two columns: the emp_id column and the skill_id column. An employee may have more than one skill, and so appear in several rows, and several employees may possess a given skill, so that the skill_id may appear several times.

The emp_skill table also has two foreign keys. The foreign key entries indicate that the emp_id column must contain a valid employee number that is a primary key in the employee table from the employee table, and that the skill_id must contain a valid entry that is a primary key in the skill table from the skill table.

A table can only have one primary key defined, but it may have as many foreign keys as necessary.

You cannot create foreign key constraints on temporary tables of any kind—local, global, or automatic.