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Please try again. The work is protected by local and international copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. You have successfully signed out and will be required to sign back in should you need to download more resources. The tags, similar to those used in HTML, define the role played by different pieces of data, much as the column headers do in the relational model. For example, the same data as in Fig. The operations on semistructured data usually involve following paths in the implied tree from an element to one or more of its nested subelements, then to subelements nested within those, and so on.
For example, starting at the outer element the entire document in Fig. Constraints on the structure of data in this model often involve the data type of values associated with a tag. For instance, are the values associated with the tag integers or can they be arbitrary character strings?
Other constraints determine which tags can appear nested within which other tags. For example, must each element have a element nested within it? W hat other tags, besides those shown in Fig. Can there be more than one genre for a movie? These and other matters will be taken up in Section A modern trend is to add object-oriented features to the relational model. There are two effects of object-orientation on relations: 1.
Values can have structure, rather than being elementary types such as integer or strings, as they were in Fig. Relations can have associated methods. We shall introduce the object-relational model in Section In these, the relation is no longer the principal data-structuring concept, but becomes only one option among many structures.
We discuss an object-oriented database model in Section 4. Its drawback was that unlike more modern models, it really operated at the physical level, which made it impossible for programmers to write code at a conveniently high level. Another such model was the network model, which was a graph-oriented, physical-level model. This difference becomes even more apparent when we discuss, as we shall, how full graph structures are embedded into tree-like, semistructured models. A brief argument follows.
Because databases are large, efficiency of access to data and efficiency of modifications to that data are of great importance. Also very important is ease of use — the productivity of programmers who use the data. Surprisingly, both goals can be achieved with a model, particularly the relational model, that: 1. Provides a simple, limited approach to structuring data, yet is reasonably versatile, so anything can be modeled.
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Provides a limited, yet useful, collection of operations on data. Together, these limitations turn into features. They allow us to implement languages, such as SQL, that enable the programmer to express their wishes at a very high level. A few lines of SQL can do the work of thousands of lines of C, or hundreds of lines of the code that had to be written to access data under earlier models such as network or hierarchical. Yet the short SQL programs, because they use a strongly limited sets of operations, can be optimized to run as fast, or faster than the code written in alternative languages.
Figure 2. In this section, we shall introduce the most important terminology regarding relations, and illustrate them with the Movies relation. Attributes appear at the tops of the columns. Usually, an attribute describes the meaning of entries in the column below. For instance, the column with attribute le n g th holds the length, in minutes, of each movie.
We show the schema for the relation with the relation name followed by a parenthesized list of its attributes. Thus, the schema for relation Movies of Fig. Thus, whenever we introduce a relation schema with a list of attributes, as above, we shall take this ordering to be the standard order whenever we display the relation or any of its rows. In the relational model, a database consists of one or more relations. The set of schemas for the relations of a database is called a relational database schema, or just a database schema.
A tuple has one component for each attribute of the relation. For instance, the first of the three tuples in Fig.
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When we wish to write a tuple 2. However, later in this book we shall talk of relations in the abstract, where the names of attributes do not matter. In th at case, we shall use single capital letters for both relations and attributes, e. For example, Gone With the Wind, , , drama is the first tuple of Fig. Notice that when a tuple appears in isolation, the attributes do not appear, so some indication of the relation to which the tuple belongs must be given. We shall always use the order in which the attributes were listed in the relation schema. It is not permitted for a value to be a record structure, set, list, array, or any other type th at reasonably can have its values broken into smaller components.
It is further assumed that associated with each attribute of a relation is a domain, th at is, a particular elementary type. The components of any tuple of the relation must have, in each component, a value that belongs to the domain of the corresponding column. For example, tuples of the Movies relation of Fig. It is possible to include the domain, or data type, for each attribute in a relation schema.
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We shall do so by appending a colon and a type after attributes. For example, we could represent the schema for the Movies relation as: Movies title:string, year:integer, length:integer, genre:string 2. Thus the order in which the tuples of a relation are presented is immaterial.
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For example, we can list the three tuples of Fig. However, when we reorder the relation schema, we must be careful to remember that the attributes are column headers. Thus, when we change the order of the attributes, we also change the order of their columns.
When the columns move, the components of tuples change their order as well. The result is that each tuple has its components permuted in the same way as the attributes are permuted. For example, Fig. We expect to insert tuples for new movies, as these appear. We also expect changes to existing tuples if we get revised or corrected information about a movie, and perhaps deletion of tuples for movies that are expelled from the database for some reason. It is less common for the schema of a relation to change. However, there are situations where we might want to add or delete attributes.
websrv2-nginx.classic.com.np/el-tesoro-escondido-de-la-santa.php Also, if we add an attribute, it may be difficult or even impossible to generate appropriate values for the new component in the existing tuples. We shall call a set of tuples for a given relation an instance of that relation. For example, the three tuples shown in Fig. We shall defer much of the discussion of constraints until Chapter 7. However, one kind of constraint is so fundamental th at we shall introduce it here: key constraints. A set of attributes forms a key for a relation if we do not allow two tuples in a relation instance to have the same values in all the attributes of the key.