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【正文】 deriving a relational database design from an ER diagram. 169。Silberschatz, Korth and Sudarshan Database System Concepts Representing Entity Sets as Tables ? A strong entity set reduces to a table with the same attributes. 169。Silberschatz, Korth and Sudarshan Database System Concepts Composite and Multivalued Attributes ? Composite attributes are flattened out by creating a separate attribute for each ponent attribute ? . given entity set customer with posite attribute name with ponent attributes firstname and lastname the table corresponding to the entity set has two attributes and ? A multivalued attribute M of an entity E is represented by a separate table EM ? Table EM has attributes corresponding to the primary key of E and an attribute corresponding to multivalued attribute M ? . Multivalued attribute dependentnames of employee is represented by a table employeedependentnames( employeeid, dname) ? Each value of the multivalued attribute maps to a separate row of the table EM ? ., an employee entity with primary key John and dependents Johnson and Johndotir maps to two rows: (John, Johnson) and (John, Johndotir) 169。Silberschatz, Korth and Sudarshan Database System Concepts Representing Weak Entity Sets ? A weak entity set bees a table that includes a column for the primary key of the identifying strong entity set 169。Silberschatz, Korth and Sudarshan Database System Concepts Representing Relationship Sets as Tables ? A manytomany relationship set is represented as a table with columns for the primary keys of the two participating entity sets, and any descriptive attributes of the relationship set. ? .: table for relationship set borrower 169。Silberschatz, Korth and Sudarshan Database System Concepts Redundancy of Tables ? Manytoone and onetomany relationship sets that are total on the manyside can be represented by adding an extra attribute to the many side, containing the primary key of the one side ? .: Instead of creating a table for relationship accountbranch, add an attribute branch to the entity set account 169。Silberschatz, Korth and Sudarshan Database System Concepts Redundancy of Tables (Cont.) ? For onetoone relationship sets, either side can be chosen to act as the ―many‖ side ? That is, extra attribute can be added to either of the tables corresponding to the two entity sets ? If participation is partial on the many side, replacing a table by an extra attribute in the relation corresponding to the ―many‖ side could result in null values ? The table corresponding to a relationship set linking a weak entity set to its identifying strong entity set is redundant. ? . The payment table already contains the information that would appear in the loanpayment table (., the columns loannumber and paymentnumber). 169。Silberschatz, Korth and Sudarshan Database System Concepts Representing Specialization as Tables ? Method 1: ? Form a table for the higher level entity ? Form a table for each lower level entity set, include primary key of higher level entity set and local attributes table table attributes person name, street, city customer name, creditrating employee name, salary ? Drawback: getting information about, ., employee requires accessing two tables 169。Silberschatz, Korth and Sudarshan Database System Concepts Representing Specialization as Tables (Cont.) ? Method 2: ? Form a table for each entity set with all local and inherited attributes table table attributes person name, street, city customer name, street, city, creditrating employee name, street, city, salary ? If specialization is total, table for generalized entity (person) not required to store information ?Can be defined as a ―view‖ relation containing union of specialization tables ?But explicit table may still be needed for foreign key constraints ? Drawback: street and city may be stored redundantly for persons who are both customers and employees 169。Silberschatz, Korth and Sudarshan Database System Concepts Relations Corresponding to Aggregation ? To represent aggregation, create a table containing ? primary key of the aggregated relationship, ? the primary key of the associated entity set ? Any descriptive attributes 169。Silberschatz, Korth and Sudarshan Database System Concepts Relations Corresponding to Aggregation (Cont.) ? . to represent aggregation manages between relationship workson and entity set manager, create a table manages(employeeid, branchname, title, managername) ? Table workson is redundant provided we are willing to store null values for attribute managername in table manages End of Chapter 2 169。Silberschatz, Korth and Sudarshan Database System Concepts ER Diagram for Exercise 169。Silberschatz, Korth and Sudarshan Database System Concepts ER Diagram for Exercise 169。Silberschatz, Korth and Sudarshan Database System Concepts ER Diagram for Exercise 169。Silberschatz, Korth and Sudarshan Database System Concepts ER Diagram for Exercise 169。Silberschatz, Korth and Sudarshan Database System Concepts Existence Dependencies ? If the existence of entity x depends on the existence of entity y, then x is said to be existence dependent on y. ? y is a dominant entity (in example below, loan) ? x is a subordinate entity (in example below, payment) loanpayment payment loan If a loan entity is deleted, then all its associated payment entities must be deleted also