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dicators supervisor and subordinate are used to create meaningful eld names in the CREATE statement for the Reports To table: CREATE TABLE Reports To ( supervisor ssn CHAR(11), subordinate ssn CHAR(11), PRIMARY KEY (supervisor ssn, subordinate ssn), FOREIGN KEY (supervisor ssn) REFERENCES Employees(ssn), FOREIGN KEY (subordinate ssn) REFERENCES Employees(ssn) ) Observe that we need to explicitly name the referenced eld of Employees because the eld name di ers from the name(s) of the referring eld(s)
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353 Translating Relationship Sets with Key Constraints
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If a relationship set involves n entity sets and some m of them are linked via arrows in the ER diagram, the key for any one of these m entity sets constitutes a key for the relation to which the relationship set is mapped Thus we have m candidate keys, and one of these should be designated as the primary key The translation discussed in Section 23 from relationship sets to a relation can be used in the presence of key constraints, taking into account this point about keys
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Consider the relationship set Manages shown in Figure 312 The table corresponding
since name ssn lot did dname budget
Employees
Manages
Departments
Key Constraint on Manages
to Manages has the attributes ssn, did, since However, because each department has at most one manager, no two tuples can have the same did value but di er on the ssn value A consequence of this observation is that did is itself a key for Manages; indeed, the set did, ssn is not a key (because it is not minimal) The Manages relation can be de ned using the following SQL statement: CREATE TABLE Manages ( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments )
A second approach to translating a relationship set with key constraints is often superior because it avoids creating a distinct table for the relationship set The idea is to include the information about the relationship set in the table corresponding to the entity set with the key, taking advantage of the key constraint In the Manages example, because a department has at most one manager, we can add the key elds of the Employees tuple denoting the manager and the since attribute to the Departments tuple This approach eliminates the need for a separate Manages relation, and queries asking for a department s manager can be answered without combining information from two relations The only drawback to this approach is that space could be wasted if several departments have no managers In this case the added elds would have to be lled with null values The rst translation (using a separate table for Manages) avoids this ine ciency, but some important queries require us to combine information from two relations, which can be a slow operation The following SQL statement, de ning a Dept Mgr relation that captures the information in both Departments and Manages, illustrates the second approach to translating relationship sets with key constraints:
The Relational Model
CREATE TABLE Dept Mgr ( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees ) Note that ssn can take on null values
This idea can be extended to deal with relationship sets involving more than two entity sets In general, if a relationship set involves n entity sets and some m of them are linked via arrows in the ER diagram, the relation corresponding to any one of the m sets can be augmented to capture the relationship We discuss the relative merits of the two translation approaches further after considering how to translate relationship sets with participation constraints into tables
354 Translating Relationship Sets with Participation Constraints
Consider the ER diagram in Figure 313, which shows two relationship sets, Manages and Works In
since name ssn lot did dname budget
Employees
Manages
Departments
Works_In
since
Manages and Works In
3
Every department is required to have a manager, due to the participation constraint, and at most one manager, due to the key constraint The following SQL statement re ects the second translation approach discussed in Section 353, and uses the key constraint: INTEGER, CREATE TABLE Dept Mgr ( did dname CHAR(20), budget REAL, ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees ON DELETE NO ACTION ) It also captures the participation constraint that every department must have a manager: Because ssn cannot take on null values, each tuple of Dept Mgr identi es a tuple in Employees (who is the manager) The NO ACTION speci cation, which is the default and need not be explicitly speci ed, ensures that an Employees tuple cannot be deleted while it is pointed to by a Dept Mgr tuple If we wish to delete such an Employees tuple, we must rst change the Dept Mgr tuple to have a new employee as manager (We could have speci ed CASCADE instead of NO ACTION, but deleting all information about a department just because its manager has been red seems a bit extreme!) The constraint that every department must have a manager cannot be captured using the rst translation approach discussed in Section 353 (Look at the de nition of Manages and think about what e ect it would have if we added NOT NULL constraints to the ssn and did elds Hint: The constraint would prevent the ring of a manager, but does not ensure that a manager is initially appointed for each department!) This situation is a strong argument in favor of using the second approach for one-to-many relationships such as Manages, especially when the entity set with the key constraint also has a total participation constraint Unfortunately, there are many participation constraints that we cannot capture using SQL-92, short of using table constraints or assertions Table constraints and assertions can be speci ed using the full power of the SQL query language (as discussed in Section 511) and are very expressive, but also very expensive to check and enforce For example, we cannot enforce the participation constraints on the Works In relation without using these general constraints To see why, consider the Works In relation obtained by translating the ER diagram into relations It contains elds ssn and did, which are foreign keys referring to Employees and Departments To ensure total participation of Departments in Works In, we have to guarantee that every did value in Departments appears in a tuple of Works In We could try to guarantee this condition by declaring that did in Departments is a foreign key referring to Works In, but this is not a valid foreign key constraint because did is not a candidate key for Works In
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