PHP 5/MySQL Programming- P81 doc

Each time you want a new relationship between an agent and a specialty, you add a new record to the agent_specialtytable.. Creating Queries That Use Link Tables Whenever you want to know

Interpreting the agent_specialty

Table with a Query

Of course, the agent_specialtytable is not directly useful to the user, because it contains nothing but foreign key references You can translate the data to some-thing more meaningful with an SQL statement:

SELECT agent_specialtyID,

agent.name AS ‘agent’, specialty.name AS ‘specialty’

FROM agent_specialty,

agent, specialty

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Specialty ID Name

Agent Specialty ID Agent ID Specialty ID

WHERE agent.agentID = agent_specialty.agentID

AND specialty.specialtyID = agent_specialty.specialtyID;

It requires two comparisons to join the three tables It is necessary to forge the

relationship between agentand agent_specialtyby common agentIDvalues It’s

also necessary to secure the bond between specialty and agent_specialty by

comparing the specialtyIDfields The results of such a query show that the

cor-rect relationships have indeed been joined, as you can see in Table 11.9

The link table provides the linkage between tables that have many-to-many

relationships Each time you want a new relationship between an agent and a

specialty, you add a new record to the agent_specialtytable

Creating Queries That Use Link Tables

Whenever you want to know about the relationships between agents and

special-ties, the data is available in the agent_specialtytable For example, if you need to

know which agents know flower arranging, you can use the following query:

SELECT

agent.name

FROM

agent,

specialty,

agent_specialty

WHERE agent.agentID = agent_specialty.agentID

AND agent_specialty.specialtyID = specialty.specialtyID

AND specialty.name = ‘Flower Arranging’;

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Agent Specialty ID Agent Specialty

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This query looks a little scary, but it really isn’t bad This query requires data from three different tables The output needs the namefrom the agent table I don’t want to remember what specialty number is associated with Flower Arranging, so

I let the query look that up from the specialtytable Since I need to know which agent is associated with a particular specialty, I use the agent_specialtytable to link up the other two tables The WHEREclause simply provides the joins

The following phrase cements the relationship between agentsand agent_specialty: agents.agentID = agent_specialty.agentID

Likewise, the following ensures the connection between specialties and agent_specialty:

agent_specialty.specialtyID = specialties.specialtyID

The last part of the WHEREclause is the actual conditional part of the query that only returns records where the specialty is flower arranging (You know, flower arrangement can be a deadly art in the hands of a skilled practitioner.)

It might be helpful to imagine the ER diagram when building queries If two tables have lines between them, use a WHEREclause to represent each line To repli-cate a one-to-many join (with one line and two tables) you need one WHEREline to handle the connection If creating a many-to-many join with a link table, you need a compound condition to handle connecting each table to the link table You can then add any other conditions that help you narrow the query

Summary

In this chapter you move beyond programming to an understanding of data, the real fuel of modern applications You learn how to take a poorly designed table and convert it into a series of well-organized tables that can avoid a lot of data problems You learn about three stages of normalization and how to build an Entity Relationship diagram You can recognize three kinds of relationships between entities and build normalized tables in SQL, including pointers for pri-mary and foreign keys You can connect normalized tables with INNER JOIN SQL statements You know how to simulate a many-to-many relationship by building

a link table The civilized world is safer for your efforts

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C H A L L E N G E S

1 Locate ER diagrams for data you work with every day (Check with your

company’s Information Technology department, for example.) Examine

these documents and see if you can make sense of them.

2 Examine a database you use regularly Determine if it follows the

require-ments stated in this chapter for a well-designed data structure If not,

explain what might be wrong with the data structure and how it could be

corrected.

3 Diagram an improved data structure for the database you examined in

Question 2 Create the required tables in SQL and populate them with

sam-ple data.

4 Design a database for data you use every day (Be warned, most data

prob-lems are a lot more complex than they first appear.) Create a data diagram;

then build the tables and populate with sample data.

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