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Coach Int Foreign Key References Member
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means that the values in the Coach field must already exist in the primary key field in the Member table; that is, there is a self relationship on the Member table. You can see the resulting lines in the diagrams in Figures 10-1 and 10-2. You can investigate all the foreign key statements in the SQL that created the tables in your database and create your own diagram, if necessary.
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The Conceptual Model vs. the Implementation
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One word of caution: there are two models you need to understand. First is the conceptual data model that describes how the data for a particular problem is interrelated. A number of methods exist for representing a conceptual model, such as entity-relationship (ER) diagrams and Unified Modeling Language (UML) class diagrams. In addition to the
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CHAPTER 10 HOW TO APPROAC H A QUERY
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conceptual model is the real database that has been implemented. The conceptual model and actual implementation may be different! The schematics that your database management software shows you depict the foreign keys that have actually been set up. However, the developer may not have implemented the foreign key constraint on the Coach field (for example) for many reasons. He may not have realized the constraint was necessary; he may not have known how to define it; or he may have decided to enforce the constraint that a coach must be an existing member some other way (with a trigger or via the interface). The CREATE statements and the relationship schematics show you what relationships have actually been implemented in the database. However, even if there is no foreign key constraint on the Coach field in the Member table, we still need to understand that members coach other members if we want to design reliable queries about coaching. It is often a good idea to sketch a conceptual model, as in Figure 10-3. Refer back to 1 if you need to refresh your understanding of how to read the lines and numbers.
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Figure 10-3. Conceptual model of the data
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The conceptual model depicts how the various bits of data actually interrelate in the real world. The database diagrams show us which foreign keys have been implemented in the database to represent the relationships. The conceptual model in Figure 10-3 and the SQL Server diagram in Figure 10-1 are just about identical (because I designed the database according to the model!). Real problems arise when the database has been designed badly (or not at all), and the implementation bears little resemblance to the reality. If we had a set of tables as in Figure 10-1 but the developer had, for some reason, chosen not to set up the foreign keys, the two models would be much the same but with a few lines missing. In that case, we could still answer questions about the data reasonably effectively (although the data values may not be very accurate or consistent).
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CH A PT ER 1 0 H O W TO A PPRO A CH A Q UERY
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In some cases, the actual database may not have much in common with the conceptual model. For example, if the database contained additional tables for coaches and managers or did not have a separate table for entries, the two models would look quite different. The likelihood of getting reliable information would be low. 11 looks at these kinds of problems, although short of a major redesign, there is not much you can do in many cases.
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What Tables Are Involved
Once you have an understanding of the tables in the database and how they are related, you can look at which tables you will need in order to extract the subset of data you require. Consider a query like Find all the men who have entered a Leeston tournament. This sentence contains a few key words. Nouns are often a clue to which tables or fields we are going to need. Verbs often help us find relationships. Let s look at the nouns. Tournament is a big clue, and we have a Tournament table, so that is a start. The word men is another noun in the query description. We don t have a Men table, but we do have a Member table. It is fairly clear then that the Member and Tournament tables are going to play a part in our query. Now we need to get a feel for how these two tables are related. Figure 10-4 shows the part of the SQL Server database diagram containing these two tables. We see that that they are not directly related but are connected via the Entry table. That makes sense, because the verb enter is in our query description.
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