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Figure 6-9. Finding teams whose managers do not play for the team
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To find all teams like this, we would say: Find the team names from all the rows (t) in the Team table where the matching row (m) in the Member table for the team manager (i.e., t.Manager = m.MemberID) has a team (m.team) that is either empty or different from the team in the Team table (m.Team <> t.TeamName). The slightly more formal calculus notation representing this situation (and illustrated in Figure 6-9) is shown in Listing 6-7.
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Listing 6-7. Calculus Expression to Find Teams Where the Manager Is Not a Member of the Team {t.TeamName | Team(t) and (m) Member(m) AND t.Manager = m.MemberID AND (m.Team IS NULL OR m.Team <> t.Team)}
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The equivalent SQL is shown in Listing 6-8. The middle two lines are equivalent to a join between the two tables on m.MemberID = t.Manager.
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Listing 6-8. SQL to Find Teams Where the Manager Is Not a Member of the Team (Based
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on Calculus)
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SELECT t.teamname FROM Member m, Team t WHERE m.MemberID = t.Manager AND (m.Team <> t.Teamname OR m.Team IS NULL)
The query could also be constructed from an algebra perspective, as shown in Listing 6-9, if you prefer this approach.
Listing 6-9. SQL to Find Teams Where the Manager Is Not a Member of the Team (Based
on Algebra)
SELECT t.teamname FROM Member m INNER JOIN Team t ON m.MemberID = t.Manager WHERE m.Team <> t.Teamname OR m.Team IS NULL
Why have we included the IS NULL condition in Listings 6-8 and 6-9 You might remember from 2 that if we make a comparison with a Null value, the result is false in SQL. If we want to find managers who aren t in a team, we need to specifically include that possibility in our query. Had the requirement been just that a manager must not belong to a different team, we could have left out the checking of Null values, because a manager with no team would have been OK. As always, clearly understanding what you are actually trying to find can be the most difficult part of formulating any query. The queries in Listings 6-8 and 6-9 will find teams with incorrect managers, but only after they have been added to the database. How do we prevent them from being added in the first place The solution depends on which database product you are using. Before changes to data are finally committed to a database, they are usually recorded in a buffer of some sort. For example, in SQL Server, the records being updated or added are kept in a temporary table called inserted. If we add or update some records to the Team table, a temporary table (inserted) that has the same structure as the Team table is created to hold the new records temporarily. We want to perform a query similar to that in Listing 6-10 to check if any of the new records have managers who don t obey the constraint. However, instead of looking at the Team table, we want to look at the records in the temporary inserted table and count how many of those are invalid.
Listing 6-10. Part of a SQL Server Trigger to Prevent Adding Invalid Team Managers IF (SELECT COUNT(*) FROM Member m INNER JOIN inserted i ON m.MemberID = i.Manager WHERE m.Team <> i.Teamname OR m.Team IS NULL) <> 0) BEGIN
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Rollback Tran END
The two innermost lines in Listing 6-10 are almost exactly the same as the query in Listing 6-8, except we have replaced Team with inserted. Rather than selecting each of the rows with incorrect managers, we have just counted how many of these rows exist. The IF statement says if there are any invalid records (COUNT() <> 0), then don t add the records to the database (that is, roll back the whole transaction). This is a bit of a crude approach, because if any of the new records are incorrect, the whole lot gets rejected. You will need to consult the documentation for your database product to see how to develop triggers that work efficiently, but the idea of using a query to check the validity of new records is a common one. In Access, the checking is done at the interface level, usually on a form. Instead of checking the inserted table as in Listing 6-9, we would create a macro to investigate the values of fields on the form before committing them to the database. Whatever the product, for a constraint of this type, we will always need to look at the new Team values and compare them with the existing information in the Member table, so a query like the one in Listing 6-8 or Listing 6-9 provides a good starting place.
Summary
There can be more than one relationship between tables. For example, a member may belong to a team is one relationship. A team has a club member who is the manager is another relationship. To find the information about a member s team (including the manager s ID) requires a join between Member and Team. If we want to also find the name of the manager, we need to join that result to another copy of the Member table, like this:
SELECT * FROM (Member m INNER JOIN Team t ON m.Team = t.TeamName) INNER JOIN Member m2 ON t.Manager = m2.MemberID
When we have two relationships between tables, there can be quite complex business rules or constraints involving the relationship, such as that the manager must be a member of the team she captains, a manager should not be a member of any team, and so on. These often require the use of triggers. The types of queries discussed in this chapter will be helpful in formulating the code required in triggers.
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