c# generate barcode Figure 10-10. We need to investigate more than one row to check both tournaments. in Font

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Figure 10-10. We need to investigate more than one row to check both tournaments.
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Where we have a question needing to satisfy both of two conditions and we need to look at more than one row in the table, we can use a self join (discussed in 5) or an intersection (discussed in 7), as in Listings 10-10 and 10-11.
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Listing 10-10. Finding Members Who Have Entered Both Tournaments Using a Self Join SELECT e1.MemberID FROM Entry e1 INNER JOIN Entry e2 ON e1.MemberID = e2.MemberID WHERE e1.TourID = 24 AND e2.TourID = 36 Listing 10-11. Finding Members Who Have Entered Both Tournaments Using an Intersection SELECT MemberID FROM Entry WHERE TourID = 24 INTERSECT SELECT MemberID FROM Entry WHERE TourID = 36
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CHAPTER 10 HOW TO APPROAC H A QUERY
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Not, Never
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Here are some examples of queries involving the word not or never : Find the members who are not seniors. Find members who are not in a team. Find members who have never played in a tournament. Often when people see not in a description of a query, they immediately think of using a Boolean NOT or a <> operator in a WHERE clause. This is fine for some queries, but not for all of them. As in the previous section, I find the following test helpful to determine the type of not query: Do I need to look at more than one row to decide if a condition is not true For the first two queries, we can look at a single row in the Member table and decide whether that member satisfies the condition. In the first query, the condition in the WHERE clause would be NOT MemberType = 'Senior' or MemberType <> 'Senior'. To find members who are not in a team, we want the Team field to be empty, so a clause like WHERE Team IS NULL would do the trick. To find the members who have not entered a tournament, what tables do we need We are certainly going to need the Entry table. We can decide if a member has entered a tournament by finding just one row with his member ID. To see if he has not entered a tournament, we need to look at every row. We also must look at the Member table to find a list of all our members. In situations like this, we need to think about the relational algebra difference operator. We can do this are by using the keyword EXCEPT (discussed in 7) or by using a nested query (discussed in 4). Two examples to retrieve the member IDs of members who have never entered a tournament are shown in Listings 10-12 and 10-13. Once we have the IDs, we can perform another join to get the names. 7 shows many examples of how to carry out difference operations.
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Listing 10-12. Finding Members Who Have Never Entered a Tournament Using EXCEPT SELECT MemberID FROM Member EXCEPT SELECT MemberID FROM Entry Listing 10-13. Finding Members Who Have Never Entered a Tournament Using a Nested Query SELECT m.MemberID FROM Member m WHERE m.MemberID NOT IN (SELECT e.MemberID FROM Entry e)
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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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Wherever you see the word all or every in a description of a query, you should immediately think of the division operator (discussed in 7). Here are some examples of such queries: Find members who have entered every open tournament. Has anyone coached all the juniors
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No Idea Where to Start
So you have looked at the query and decided which tables you think will be involved. You re not sure if a join is the right path. You ve checked for some key words, but you still feel confused. Now what This is not uncommon (it happens to me regularly), so just relax. When I have no idea where to start, I forget all about set operations and SQL. I stop thinking about tables, foreign keys, joins, and so on. Instead, I open the tables I think I will need to answer the question and look at some of the data. I try to find examples that should be retrieved by the query. Then I try to write down the conditions that make that particular data acceptable. This is the relational calculus approach. Relational calculus is describing what the rows returned by the query are like. I ve been using this approach all the way through the book, alongside the algebra approach of deciding how to manipulate the tables. Let s try a query that stumped me a bit when I first thought of it: Which teams have a coach as a manager The steps described here can really help.
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