create barcode image c# SQL to Find Members Who Have Entered Both Tournaments 24 and 36 in Font

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Listing 5-14. SQL to Find Members Who Have Entered Both Tournaments 24 and 36
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(Using a Self Join)
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SELECT e1.MemberID FROM Entry e1 INNER JOIN Entry e2 ON e1.MemberID = e2.MemberID WHERE e1.TourID = 24 AND e2.TourID = 36
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If you compare Listings 5-13 and 5-14, you will see how similar they are. They will both produce exactly the same result. You will probably find one or other to be more intuitive.
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Many queries require us to obtain information from two rows of a table. This turns up in a number of situations. The main ones are where we have self relationships or questions involving the word both.
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Self Relationships
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We have a self relationship when different instances of a class are related to each other. In the example in this chapter, we had that members are coaches of other members. Queries about coaches or coaching relationships require self joins, which take two copies of the table and join them. The self join to provide the names of members and their coaches follows. The copy with the information about the member has the alias m, and the copy with information about the coach has the alias c.
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SELECT m.LastName, m.FirstName, c.LastName, c.FirstName FROM Member m INNER JOIN Member c ON m.Coach = c.MemberID
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C HA PTER 5 S ELF JOINS
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Questions Involving the Word Both
Questions with the word both often mean we need to look at two rows in a table. In our example, we wanted to find the MemberID of members who have entered both tournaments 24 and 36. We needed to find two rows in the Entry table (e1 and e2) for that member: one for tournament 24 and the other for tournament 36. The following is the calculus-based SQL statement:
SELECT e1.MemberID FROM Entry e1, Entry e2 WHERE e1.MemberID = e2.MemberID AND e1.TourID = 24 AND e2.TourID = 36
This statement is equivalent to a self join between two copies of the Entry table (on e1.MemberID = e2.MemberID), followed by a select condition to find the rows for the two tournaments.
More Than One Relationship Between Tables
n order to get correct information from your database, it is essential that the design is appropriate and you understand it properly. You have already seen simple relationships between tables (for example, each member is associated with one member type), and in 5, we looked at self relationships (for example, members may coach other members). Another situation that occurs frequently is where there is more than one relationship between two tables.
Representing Multiple Relationships Between Tables
Let s look at the model in Figure 6-1, which shows how we might incorporate the idea of teams into our club database. The top line in Figure 6-1 can be interpreted, from left to right, as that a particular member might manage (at most) one team; and from right to left, as that each team has exactly one manager. The bottom relationship means, from left to right, that a particular member might play in (at most) one team; and from right to left, that a team has at least one member and could have many members playing for it.
Figure 6-1. Two relationships between the Member and Team classes
CHAPTER 6 MORE THAN ONE RELATION SHIP BE TWEEN TA BLES
We can represent this model by introducing a new table, Team, into our database. The top relationship can be represented by including a foreign key field Manager in the Team table, and the bottom relationship can be represented by including a foreign key field Team in the Member table. Some sample rows from the two tables (with some of the columns in Member hidden) are shown in Figure 6-2.
Member
Team
Figure 6-2. Foreign keys Team in Member table and Manager in Team table to represent the
relationships in Figure 6-1
From the Member table, we can see that four people play for TeamB (Brenda Nolan, William Cooper, Robert Pollard, and Betty Young) and from the Team table, we can see that member 153 (Brenda Nolan) is the manager of TeamB. The eagle-eyed will notice that there is nothing in the data model that says whether or not a manager must be a member of the team. TeamB s manager is a member of TeamB, whereas TeamA s manager 239 (Thomas Spence) is not a member of TeamA. The only constraints implied by the foreign keys are that the manager of a team must be in the Member table and a member can belong only to a team that exists in the Team table.
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