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One major benefit of using a dedicated database system is getting support for relational databases. A relational database is a database that s comprised of data grouped into one or more tables that can be linked together. A table stores information about one type of thing. For example, an address book database might be made up of a people table, an addresses table, and a phonenumbers table. Each table stores information about people, addresses, and phone numbers, respectively. The people table would likely have a number of attributes (known as columns, in database land) such as name, age, and gender. Each row of the table that is, an individual person would then have information in each column. Figure 9-1 shows an example.

Loaded suite test/functional/admin/author_controller_test Started .. Finished in 0.171436 seconds. 2 tests, 12 assertions, 0 failures, 0 errors The results look good. We can now point our browser to http://localhost:3000/admin/ author/new and try to create a new author. Leaving the last name blank, we re greeted with the response shown in Figure 2-2 when the form is submitted.

First we define the messages that the ChatServer3 will accept or send:

Figure 9-1 s example also includes a column called id. In relational databases it s standard procedure to have an id column on most tables to identify each row uniquely. Although you could look up and retrieve data based on other columns, such as name, numeric IDs are useful when you re creating relationships between tables.

Note In Figure 9-1, the table headings are written in a typical style, as you d expect in a normal address

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case case case case object GetMessages class Messages(msg: List[String]) class Remove(who: Actor) class Add(who: Actor)

book or spreadsheet. However, when dealing with relational databases at a lower level, it s reasonably common to use all lowercase names for column and table names. This explains why the text, and later code examples, in this chapter, will refer to table and column names in lowercase only.

Figure 2-2. Failing creation of a new author Seems that our validations are working, and as the tests agree, we move on to implement editing of existing authors.

One benefit of relational databases is the way rows in different tables can be related to one another. For example, your people table could have an address_id column that stores the ID of the address associated with this user. If you want to find out the address of a particular person, you can look up his or her address_id, and then look up the relevant row of the addresses table. The reason for this sort of relationship is that many people in your people database might share the same address, and rather than store the address separately for each person, it s more efficient to store a reference instead. This also means that if you update the address in the future, it updates for all the relevant users at the same time. Relationships also make it possible to do many-to-many relationships. You could create a separate table called related_people that has two columns, first_person_id and second_person_id. This table could store pairs of ID numbers that signify two people are related to each other. To work out to whom a person is related, you can simply look for any rows mentioning his or her ID number and you d get back the ID numbers of that person s related people. This sort of relationship is used in most databases and is what makes relational databases so useful.

And we define the chat server object:

Four well-known relational database systems available today that work on both Windows and Unix operating systems are MySQL, PostgreSQL, Oracle, and SQLite. Each has significantly different features from the others, and therefore has different uses.

object ChatServer3 extends Actor { private var chats: List[String] = Nil private var listeners: List[Actor] = Nil