Intersection of Two Tables in SQL

Question

Basically I need to create a new table that uses specific information from two other tables.

For example, I have a table called person with the elements person_id, first_name, last_name, gender, age, and fav_quote. I have a second table called department with the elements dept_id, dept_name, and building. I now need to create and intersection table with the person_id and dept_id elements included. And both must be the primary key (which I assume just means PRIMARY KEY (person_id, dept_id) command in my source).

CREATE TABLE person (

person_id INT(8) NOT NULL auto_increment,

first_name VARCHAR(25) NOT NULL,

last_name VARCHAR(25) NOT NULL,

gender VARCHAR(1),

age INT(8),

fav_quote TEXT,

PRIMARY KEY (person_id)

);

CREATE TABLE department (

dept_id INT(8) NOT NULL auto_increment,

dept_name VARCHAR(25) NOT NULL,

building VARCHAR(25) NOT NULL,

PRIMARY KEY (dept_id)

);

That is the code I have for the initial two tables I'm just not sure how to create an intersection and, having gone back over my notes, I can't find the instructions on how to write it.

Answer 1

You got the primary key part right. I'd add foreign keys to your existing table in order to prevent creating interactions with people or departments that don't exist:

CREATE TABLE person_department
    person_id INT(8) NOT NULL,
    dept_id INT(8) NOT NULL,
    PRIMARY KEY(person_id, dept_id),
    FOREIGN KEY(person_id) REFERENCES person(person_id),
    FOREIGN KEY(dept_id) REFERENCES department(dept_id)
)

Answer 2

You need a table with 2 fields; person_id and dept_id. The table will have foreign keys to the two tables person and department primary keys' and a composite primary key of both.

Also, this table is only necessary if there is a one to many relationship of person to department. Otherwise just add dept_id as a foreign key in person.