Question

In class, we are all ‘studying’ databases, and everyone is using Access. Bored with this, I am trying to do what the rest of the class is doing, but with raw SQL commands with MySQL instead of using Access.

I have managed to create databases and tables, but now how do I make a relationship between two tables?

If I have my two tables like this:

CREATE TABLE accounts(     account_id INT NOT NULL AUTO_INCREMENT,     customer_id INT( 4 ) NOT NULL ,     account_type ENUM( 'savings', 'credit' ) NOT NULL,     balance FLOAT( 9 ) NOT NULL,     PRIMARY KEY ( account_id ) ) 

and

CREATE TABLE customers(     customer_id INT NOT NULL AUTO_INCREMENT,     name VARCHAR(20) NOT NULL,     address VARCHAR(20) NOT NULL,     city VARCHAR(20) NOT NULL,     state VARCHAR(20) NOT NULL,     PRIMARY KEY ( customer_id ) ) 

How do I create a ‘relationship’ between the two tables? I want each account to be ‘assigned’ one customer_id (to indicate who owns it).

Answer 1

If the tables are innodb you can create it like this:

CREATE TABLE accounts(     account_id INT NOT NULL AUTO_INCREMENT,     customer_id INT( 4 ) NOT NULL ,     account_type ENUM( 'savings', 'credit' ) NOT NULL,     balance FLOAT( 9 ) NOT NULL,     PRIMARY KEY ( account_id ),      FOREIGN KEY (customer_id) REFERENCES customers(customer_id)  ) ENGINE=INNODB; 

You have to specify that the tables are innodb because myisam engine doesn’t support foreign key. Look here for more info.