How to create relationships in MySQL

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 )
)

​x
 
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 )
)

 
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

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;

 
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.

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Answer