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Chapter 7 The Relational Data Model
Cite this Email this Add to favourites Print this page. Each employee and department has a number and a name. You could organize this information as shown in Table 1. For example, deleting all the employees in the Purchasing department will eliminate the department itself. By using this structure, you can examine the EMP table to find out that Doug works in department You might think that Table 1 looks more efficient. However, retrieving the information you need in a number of different ways is much easier with the two-table structure.
Joining the information in the two tables for more efficient retrieval is exactly the problem that relational databases were designed to solve. When the tables are implemented in the database, the information in the two tables is linked by using special columns called foreign keys. In the example, the DeptNo column is the foreign key linking the Department and Employee tables.
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A purchase order can include many items. Table 5 shows that Purchase Order includes three separate items.
The link foreign key between the tables is the Purchase Order Number. Both PKs and AKs have the ability to uniquely identify a row within a table. Additional technology may be applied to ensure a unique ID across the world, a globally unique identifier , when there are broader system requirements. The primary keys within a database are used to define the relationships among the tables. When a PK migrates to another table, it becomes a foreign key in the other table. When each cell can contain only one value and the PK migrates into a regular entity table, this design pattern can represent either a one-to-one or one-to-many relationship.
Most relational database designs resolve many-to-many relationships by creating an additional table that contains the PKs from both of the other entity tables—the relationship becomes an entity; the resolution table is then named appropriately and the two FKs are combined to form a PK. The migration of PKs to other tables is the second major reason why system-assigned integers are used normally as PKs; there is usually neither efficiency nor clarity in migrating a bunch of other types of columns. Relationships are a logical connection between different tables, established on the basis of interaction among these tables.
Often procedures can be used to greatly reduce the amount of information transferred within and outside of a system. For increased security, the system design may grant access to only the stored procedures and not directly to the tables. Fundamental stored procedures contain the logic needed to insert new and update existing data. More complex procedures may be written to implement additional rules and logic related to processing or selecting the data.
A relational database has become the predominant type of database. Other models besides the relational model include the hierarchical database model and the network model. The table below summarizes some of the most important relational database terms and the corresponding SQL term:. A relation is defined as a set of tuples that have the same attributes. A tuple usually represents an object and information about that object. Objects are typically physical objects or concepts. A relation is usually described as a table , which is organized into rows and columns.
All the data referenced by an attribute are in the same domain and conform to the same constraints. The relational model specifies that the tuples of a relation have no specific order and that the tuples, in turn, impose no order on the attributes. Applications access data by specifying queries, which use operations such as select to identify tuples, project to identify attributes, and join to combine relations. Relations can be modified using the insert , delete , and update operators. New tuples can supply explicit values or be derived from a query. Similarly, queries identify tuples for updating or deleting.
Tuples by definition are unique. If the tuple contains a candidate or primary key then obviously it is unique; however, a primary key need not be defined for a row or record to be a tuple. The definition of a tuple requires that it be unique, but does not require a primary key to be defined. Because a tuple is unique, its attributes by definition constitute a superkey. In a relational database, all data are stored and accessed via relations. Relations that store data are called "base relations", and in implementations are called "tables". Other relations do not store data, but are computed by applying relational operations to other relations.
These relations are sometimes called "derived relations". In implementations these are called " views " or "queries". Derived relations are convenient in that they act as a single relation, even though they may grab information from several relations. Also, derived relations can be used as an abstraction layer. A domain describes the set of possible values for a given attribute, and can be considered a constraint on the value of the attribute. Mathematically, attaching a domain to an attribute means that any value for the attribute must be an element of the specified set.
The character string "ABC" , for instance, is not in the integer domain, but the integer value is. Another example of domain describes the possible values for the field "CoinFace" as "Heads","Tails".go to site
The New Relational Database Dictionary: Terms, Concepts, and Examples - Free PDF Download
So, the field "CoinFace" will not accept input values like 0,1 or H,T. Constraints make it possible to further restrict the domain of an attribute. For instance, a constraint can restrict a given integer attribute to values between 1 and Constraints provide one method of implementing business rules in the database and support subsequent data use within the application layer. SQL implements constraint functionality in the form of check constraints.
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Constraints restrict the data that can be stored in relations. These are usually defined using expressions that result in a boolean value, indicating whether or not the data satisfies the constraint. Constraints can apply to single attributes, to a tuple restricting combinations of attributes or to an entire relation.
Since every attribute has an associated domain, there are constraints domain constraints. The two principal rules for the relational model are known as entity integrity and referential integrity. A primary key uniquely specifies a tuple within a table. In order for an attribute to be a good primary key it must not repeat. While natural attributes attributes used to describe the data being entered are sometimes good primary keys, surrogate keys are often used instead.