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When we talk about a database, we must differentiate between the database schema, which is the logical design of the database, and a database instance, which is a snapshot of the data in the database at a given instant in time The concept of a relation corresponds to the programming-language notion of a variable The concept of a relation schema corresponds to the programming-language notion of type de nition It is convenient to give a name to a relation schema, just as we give names to type de nitions in programming languages We adopt the convention of using lowercase names for relations, and names beginning with an uppercase letter for relation schemas Following this notation, we use Account-schema to denote the relation schema for relation account Thus, Account-schema = (account-number, branch-name, balance) We denote the fact that account is a relation on Account-schema by account(Account-schema) In general, a relation schema consists of a list of attributes and their corresponding domains We shall not be concerned about the precise de nition of the domain of each attribute until we discuss the SQL language in 4 The concept of a relation instance corresponds to the programming language notion of a value of a variable The value of a given variable may change with time; similarly the contents of a relation instance may change with time as the relation is updated However, we often simply say relation when we actually mean relation instance As an example of a relation instance, consider the branch relation of Figure 33 The schema for that relation is Branch-schema = (branch-name, branch-city, assets) Note that the attribute branch-name appears in both Branch-schema and Accountschema This duplication is not a coincidence Rather, using common attributes in relation schemas is one way of relating tuples of distinct relations For example, suppose we wish to nd the information about all of the accounts maintained in branches
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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The McGraw Hill Companies, 2001
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branch-name Brighton Downtown Mianus North Town Perryridge Pownal Redwood Round Hill Figure 33
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branch-city Brooklyn Brooklyn Horseneck Rye Horseneck Bennington Palo Alto Horseneck
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The branch relation
located in Brooklyn We look rst at the branch relation to nd the names of all the branches located in Brooklyn Then, for each such branch, we would look in the account relation to nd the information about the accounts maintained at that branch This is not surprising recall that the primary key attributes of a strong entity set appear in the table created to represent the entity set, as well as in the tables created to represent relationships that the entity set participates in Let us continue our banking example We need a relation to describe information about customers The relation schema is Customer -schema = (customer-name, customer-street, customer-city) Figure 34 shows a sample relation customer (Customer-schema) Note that we have omitted the customer-id attribute, which we used 2, because now we want to have smaller relation schemas in our running example of a bank database We assume that the customer name uniquely identi es a customer obviously this may not be true in the real world, but the assumption makes our examples much easier to read customer-name Adams Brooks Curry Glenn Green Hayes Johnson Jones Lindsay Smith Turner Williams Figure 34 customer-street customer-city Spring Pittsfield Senator Brooklyn North Rye Sand Hill Woodside Walnut Stamford Main Harrison Alma Palo Alto Main Harrison Park Pittsfield North Rye Putnam Stamford Nassau Princeton The customer relation
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
I Data Models
3 Relational Model
The McGraw Hill Companies, 2001
3
Relational Model
In a real-world database, the customer-id (which could be a social-security number, or an identi er generated by the bank) would serve to uniquely identify customers We also need a relation to describe the association between customers and accounts The relation schema to describe this association is Depositor -schema = (customer-name, account-number) Figure 35 shows a sample relation depositor (Depositor-schema) It would appear that, for our banking example, we could have just one relation schema, rather than several That is, it may be easier for a user to think in terms of one relation schema, rather than in terms of several Suppose that we used only one relation for our example, with schema (branch-name, branch-city, assets, customer-name, customer-street customer-city, account-number, balance) Observe that, if a customer has several accounts, we must list her address once for each account That is, we must repeat certain information several times This repetition is wasteful and is avoided by the use of several relations, as in our example In addition, if a branch has no accounts (a newly created branch, say, that has no customers yet), we cannot construct a complete tuple on the preceding single relation, because no data concerning customer and account are available yet To represent incomplete tuples, we must use null values that signify that the value is unknown or does not exist Thus, in our example, the values for customer-name, customer-street, and so on must be null By using several relations, we can represent the branch information for a bank with no customers without using null values We simply use a tuple on Branch-schema to represent the information about the branch, and create tuples on the other schemas only when the appropriate information becomes available In 7, we shall study criteria to help us decide when one set of relation schemas is more appropriate than another, in terms of information repetition and the existence of null values For now, we shall assume that the relation schemas are given We include two additional relations to describe data about loans maintained in the various branches in the bank: customer-name Hayes Johnson Johnson Jones Lindsay Smith Turner Figure 35 account-number A-102 A-101 A-201 A-217 A-222 A-215 A-305
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