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BUILDING A RELATIONAL DATABASE FROM THE DATA MODEL The data model comprises the conceptual schema, or the description of the structure of the database. This is one of three schemas, or designs that database developers refer to. The other schemas include the external schema, which is the database as conceived by the end-users, and the internal schema, which is the set of actual file structures on disk used by the database management system (SQL Server, Oracle, etc.). With the conceptual schema created, the next task is to convert the data model into tables, relationships, and data integrity rules. An entity type is represented as a table, where each row in the table represents an instance of that entity type. In relational database terminology, a table is called a relation. Note that a relation is a table, not a relationship. Later we will also create means to represent relationships. A relation consists of rows, each of which represents an instance of the entity type, and of columns, each of which represents one of the attributes of the entity type. Each row of a relation is called a tuple. Practitioners use the word row more often than tuple, but a tuple is a row of a table, an instance of a relation, an instance of an entity type. Each tuple consists of the values of the attributes for that instance of the entity type. Another word for attribute is field. So, in discussions of relational databases, you must keep in mind these synonyms: relation and table, tuple and row, attribute and field. The first step is to create a relation for each strong entity in the data model. Each of the attributes of the entity type in the data model will become a column name in the relation. At this time one must choose an attribute, or set of attributes, called a primary key, which will uniquely identify each row of the relation. The ideal key is short, numeric, and never-changing. The ideal is not always possible to achieve, but it can be helpful to keep the ideal in mind when choosing a key. For instance, if the Student table includes the attributes of name, address, and social security number (SSN), in addition to other attributes, one could probably choose the combination name address, or the single attribute SSN, to uniquely identify students. Choosing SSN would be wiser, because SSN will be more efficient to process, due to its numeric type, and it will change even less frequently than a name or an address. Sometimes there is no obviously good key attribute among the attributes of the table. One choice is to concatenate the values of several fields to achieve an identifier that will be unique for each row. If this approach leads to long, alphanumeric keys, it can be better to use a surrogate key. A surrogate key is simply a number, generated by the DBMS, which is assigned to each tuple. In the case of the Student table, if SSN were not one of the attributes to be stored for each student, one might decide to generate a surrogate key for the Student table and call it StudentID . The second step is to create a relation for each weak and ID-dependent entity type. As with strong entities, each attribute of the entity type in the data model becomes a column in the new relation. In addition, one must add a column to the weak or ID-dependent relation that will hold the foreign key of the strong entity tuple to which it is related. A foreign key is a column in a relation, which establishes a relationship with data in another relation. For instance, suppose our data model includes entity type StudentComputer , and that StudentComputer is a weak entity associated with Student . That is, the database will track the information about each student s computer only as long as the student is part of the database. In addition to attributes of the student s computer such as make and serial number, the StudentComputer relation will have a column identifying the student who owns the computer. If SSN is the key of the Student relation, the foreign key in the StudentComputer relation will contain values of student social security numbers. It is not necessary for the column names in the two relations to be the same. Thus, even though the key column of the Student relation is named SSN , the foreign key column in StudentComputer might be called StudentSSN . The new relation created for the weak entity must also have a primary key of its own. Choosing the primary key for the weak entity relation involves the same considerations as choosing the primary key for a strong entity relation. If the weak entity is ID-dependent on the strong entity, then make the key of the ID-dependent relation a combination of the foreign key field, and one or more other attributes of the ID-dependent relation. Another application of ID-dependent entities is in modeling multivalued attributes. For instance, one may want to provide for multiple addresses for each student; many will have one address during the academic year, and another during the summer, for instance. In such a case, model an ID-dependent entity called Address , and create a relation with attributes such as Street , City , State , etc., as well as a foreign key attribute that will hold values of the primary key for the Student relation.
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