Many-to-Many Relationships in Software

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The next stop on our journey though the world of data modeling problems is the big, bad many-to-many relationship Once you have a handle on how to physically implement a many-to-many relationship, working with them isn t very difficult What problems are there that you should be aware of when using many-to-many relationships Often, the fact that you have implemented a pure many-to-many relationship is, in and of itself, the problem When we say a pure many-to-many relationship, we mean one in which the join table is made up only of the foreign keys that point to the other tables For example, look at the many-to-many relationship shown in Figure 89
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FIGURE 89 A pure many-to-many relationship In this example, we have two tables Tickets and Queue with a many-to-many relationship between them, implemented here using TicketQueue as a join table Not shown are the other attributes of these three tables Both Tickets and Queue contain other elements in addition to their primary keys In this case, the join table, TicketQueue, exists only to define the relationship; it does not add any further information about tickets or queues At first glance, you might think this relationship is just fine, but let s take a moment and think about what we are modeling We are trying to
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model a relationship whereby tickets are placed into work queues Each queue can have multiple tickets, and each ticket can be in multiple queues; so far, our many-to-many is looking OK What you have to ask yourself in this situation is, Are there any other attributes we should have in the join table to complete the picture of what we are modeling In this example, there indeed are other helpful attributes that we could add For example, we might add an InsertedTime column to the TicketQueue table to track when the ticket was placed on the queue There are perfectly valid uses for many-to-many relationships, but if you look closely at your model, you will find that there are other attributes that you can add to the join tables Don t get us wrong; we are not saying that many-to-many relationships are bad, but they do add the extra overhead of having to bring a third table into a relationship We are big fans of reducing the number of tables to increase performance Properly evaluating your many-to-many relationships is a big step toward a clean, wellperforming database model
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In this short chapter, we ve covered some of the common problems that you may run into when building your data model By making yourself aware of these issues you should be able to plan accordingly and avoid these mistakes Once a data model has been completed physically implemented and you start using the database in a production environment, making changes is often difficult at best We always want to build good, sound models, but sometimes we fall into these simple traps Hopefully, you now have the tools you need to build models without building in these little headaches
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CREATING THE PHYSICAL MODEL
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9 Creating the Physical Model with SQL Server 10 Indexing Considerations 11 Creating an Abstraction Layer in SQL Server
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CREATING THE PHYSICAL MODEL WITH SQL SERVER
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We have come a long way since we started our discussion about data modeling, but now we are on the downhill slide You have done all the hard work gathering requirements, transforming them into a logical model, and acquiring customer approval of that model Throughout this process you had to make sure to avoid common problems and keep everything normalized to a reasonable level Now it s time to use the logical model and the business requirements to come up with your physical model This physical model will become your actual database in SQL Server This chapter first covers the naming guidelines we suggest for SQL Server objects, and then we talk about creating your physical model So without further ado, let s get right into creating our physical model in Microsoft SQL Server
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