PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

Foreign Keys
315
You should see the order displayed:
+---------+------------+--------+------------+
| orderid | customerid | amount | date |
+---------+------------+--------+------------+
| 5 | 2 | 69.98 | 2008-06-18 |
+---------+------------+--------+------------+
Leaving this connection open, go to your other connection and run the same select
query.You should not be able to see the order:
Empty set (0.00 sec)
(If you can see it, most likely you forgot to turn off autocommitting. Check this and that
you converted the table in question to the InnoDB format.) The reason is that the transaction
has not yet been committed. (This is a good illustration of transaction isolation in
action.)
Now go back to the first connection and commit the transaction:
commit;
You should now be able to retrieve the row in your other connection.
Foreign Keys
InnoDB also supports foreign keys.You may recall that we discussed the concept of foreign
keys in Chapter 8,“Designing Your Web Database.”When you use MyISAM tables,
you have no way to enforce foreign keys.
Consider, for example, inserting a row into the order_items table.You need to include
a valid orderid. Using MyISAM, you need to ensure the validity of the orderid you
insert somewhere in your application logic. Using foreign keys in InnoDB, you can let
the database do the checking for you.
How do you set this up? To create the table initially using a foreign key, you could
change the table DDL statement as follows:
create table order_items (
orderid int unsigned not null references orders(orderid),
isbn char(13) not null,
quantity tinyint unsigned,
primary key (orderid, isbn)
) type=InnoDB;
We added the words references orders(orderid) after orderid.This means this column
is a foreign key that must contain a value from the orderid column in the orders
table.
Finally, we added the table type type=InnoDB at the end of the declaration.This is
required for the foreign keys to work.