create barcode image c# Algebra to Retrieve Subset of Information from the View AllTourInfo in Font

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Listing 3-12. Algebra to Retrieve Subset of Information from the View AllTourInfo
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LastName, FirstName ( Year=2006 AND Type= Open (AllTourInfo))
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Listing 3-13. SQL to Retrieve Subset of Information from the View AllTourInfo SELECT LastName, FirstName FROM AllTourInfo WHERE TourType = 'Open' AND Year = 2006
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If we didn t want to have the intermediate step of creating a view, we could combine all the operations into one SQL query as in Listing 3-14 where the name of the view has been replaced with the SQL SELECT statement, Listing 3-11, that we used to define it.
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Listing 3-14. SQL to Retrieve Information from Original Tables SELECT LastName, FirstName FROM ( Member m INNER JOIN Entry e ON m.MemberID = e.MemberID) INNER JOIN Tournament t ON e.TourID = t.TourID WHERE TourType = 'Open' AND Year = 2006
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Order of Algebra Operations
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In our relational algebra description, we joined all the tables first and then selected the appropriate rows and columns. The result of the join is an intermediate table (as in Figure 3-7) that is potentially extremely large if there are lots of members and tournaments. We could have done the algebra in a different order. We could have first selected just the Open tournaments from the Tournament table and the 2006 tournaments from the
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C HA P TER 3 A F IRS T LO O K A T JOI NS
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Entry tables, as shown in Figure 3-8. Joining these two smaller tables with each other and then joining that result with Member would result in a much smaller intermediate table.
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Selecting 2006 entries
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Select Open tournaments
Figure 3-8. Selecting rows from the Entry and Tournament tables before joining them
So, should we worry about the order of the operations The answer is yes order of operation makes a huge difference but if you are using SQL, then it is not your problem to worry about it. The SQL statement is always going to look like the one in Listing 3-14 (but with the tables possibly in a different order). The SQL statement is sent to the engine of whatever database program you are using, and the query will be optimized. This means the database program figures out the best order to do things. Some products do this extremely well, and others not so well. Many products have analyzer tools that will let you see in what order things are being done. For many queries, writing your SQL differently doesn t make much difference, but you can make things more efficient by providing indexes for your tables. We will look at these issues more carefully in 9.
A Calculus Approach
The reason that the way we write our SQL statements often doesn t affect the efficiency of a query is that SQL is fundamentally based on relational calculus. The original SQL standards did not even have algebra keywords like INNER JOIN. SQL statements without these algebra keywords describe what the retrieved rows should be like, so they do not have anything to say about how. Let s look at a calculus approach to our question Who entered Open tournaments in 2006 We want to just retrieve some names from the Member table. Forget joins, and think how you would know whether a particular name should be retrieved if you were shown just the three tables and knew nothing about databases or foreign keys or anything. Imagine a finger m tracing down the table as in Figure 3-9.
CHAPTER 3 A FIRST LOOK AT JOIN S
t m e
a) Member (some columns)
b) Entry
c) Tournament
Figure 3-9. Using row variables to describe the rows that satisfy the query conditions
Do we want to write out William Cooper, the name to which m is currently pointing How would we know Well, first we have to find a row with his ID (235) in the Entry table for the year 2006 such as the one where finger e is pointing (where TourID is 40). Then we have to find a row with that tournament ID (40) in the Tournament table and check whether it is an Open tournament. Looking at Figure 3-9, we see that the rows where the three fingers are pointing give us enough information to know that William Cooper did indeed enter an Open tournament in 2006. This set of conditions describes what a row in the result table should be like. Now let s write that last paragraph a bit more succinctly. Read the following sentence with reference to the rows denoted in Figure 3-9: I ll write out the names from row m, where m comes from the Member table, if there exists a row (e) in the Entry table where m.MemberID is the same as e.MemberID and e.Year is 2006 and there also exists a row (t) in the Tournament table where e.TourID is the same as t.TourId and t.TourType has the value Open . And now (if you prefer), Listing 3-15 shows the same sentence represented in calculus notation. The construction (e) means There exists a row e, and Entry(e) means where e comes from the Entry table.
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