c# generate barcode Figure 10-4. Part of the database diagram showing the Member and Tournament tables in Font

Create PDF417 in Font Figure 10-4. Part of the database diagram showing the Member and Tournament tables

Figure 10-4. Part of the database diagram showing the Member and Tournament tables
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So it looks like at least three tables will be involved in our query: Member, Tournament, and Entry. We then use our understanding of the relational algebra to decide how these tables need to be combined. Do we need a join or a union, or some combination of these and other relational operators We ll look at ways to help decide on the appropriate operations in later sections in this chapter.
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CHAPTER 10 HOW TO APPROAC H A QUERY
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Look at Some Data Values
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Requests for information from a database are usually couched in rather informal and imprecise natural language. Even a simple request, such as Find all the men who have entered a Leeston tournament, has a few things we need to clarify. Having a look at the actual data in the tables can sometimes help. Our query does not actually find the men, but returns some information about them. Looking at the data values in the table will help us decide what information might be helpful. Presumably, the questioner would like to see the names of the men. Do we need the IDs as well We will need IDs if we want to distinguish two members with the same name. It may not always be clear what some of the words in the question refer to. What is a Leeston tournament Is Leeston the name of a tournament, a type of tournament, or a location Looking at a few rows of the Tournament table (as shown later in Figure 10-5) can help us. We see that the TourName field has the value Leeston here and there. Sometimes it might not be so easy to determine what imprecise words in the query description refer to. It may be necessary to talk to the developer or users to get a better understanding of what information they are trying to retrieve. How do we determine which members are men Fortunately, the Member table has a Gender column, and it looks like we want M values. Is selecting rows with values of M going to be enough Might there be some rows that have m or male as the values We ll look at how to deal with issues of inconsistent data in the next chapter. For now, let s assume that men are denoted by M values. For the simple query in this example, we now have a more precise description. It is something like Retrieve the MemberID, LastName, and FirstName of the men (Gender = 'M') who have entered the tournament where the value of TourName is Leeston. If you are on the ball, you might think of some other particulars that need clearing up. It is often a good idea to ask why this information is required. Do we just want to find which men have ever been to Leeston, or do we want to know how many times our male club members have entered Leeston tournaments These questions can have different answers, as you will see in the Retain the Appropriate Columns section coming up soon.
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Big Picture Approach
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My first attempt at a query is seldom elegant or complete. For a simple query like Find all the men who have entered a Leeston tournament, there are two ways I might tackle it, depending how my muses are working. One way is the big picture. I do this if I have a bit of an idea of how to combine the tables. I will cover the other way in the section No Idea Where to Start which I use when I have no idea how to start! In the big picture approach, I like to combine all the tables I ll need and retain all the columns, so I can see what is happening. I usually find it easiest to have an SQL window of
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