Data Models, Schemas, and Instances
One fundamental characteristic of the database approach is that it provides some level of data abstraction. Data abstraction generally refers to the suppression of details of data organization and storage, and the highlighting of the essential features for an improved understanding of data. One of the main characteristics of the database approach is to support data abstraction so that different users can perceive data at their preferred level of detail. A data model—a collection of concepts that can be used to describe the structure of a database—provides the necessary means to achieve this abstraction. By structure of a database we mean the data types, relationships, and constraints that apply to the data. Most data models also include a set of basic operations for specifying retrievals and updates on the database.
In addition to the basic operations provided by the data model, it is becoming more common to include concepts in the data model to specify the dynamic aspect or behavior of a database application. This allows the database designer to specify a set of valid user-defined operations that are allowed on the database objects. An example of a user-defined operation could be COMPUTE_GPA, which can be applied to a STUDENT object. On the other hand, generic operations to insert, delete, modify, or retrieve any kind of object are often included in the basic data model operations. Concepts to specify behavior are fundamental to object-oriented data models (see Chapter 11) but are also being incorporated in more traditional data models. For example, object-relational models (see Chapter 11) extend the basic relational model to include such concepts, among others. In the basic relational data model, there is a provision to attach behavior to the relations in the form of persistent stored modules, popularly known as stored procedures (see Chapter 13).
1. Categories of Data Models
Many data models have been proposed, which we can categorize according to the types of concepts they use to describe the database structure. High-level or conceptual data models provide concepts that are close to the way many users per-ceive data, whereas low-level or physical data models provide concepts that describe the details of how data is stored on the computer storage media, typically magnetic disks. Concepts provided by low-level data models are generally meant for computer specialists, not for end users. Between these two extremes is a class of representational (or implementation) data models, which provide concepts that may be easily understood by end users but that are not too far removed from the way data is organized in computer storage. Representational data models hide many details of data storage on disk but can be implemented on a computer system directly.
Conceptual data models use concepts such as entities, attributes, and relationships. An entity represents a real-world object or concept, such as an employee or a project from the miniworld that is described in the database. An attribute represents some property of interest that further describes an entity, such as the employee’s name or salary. A relationship among two or more entities represents an association among the entities, for example, a works-on relationship between an employee and a project. Chapter 7 presents the Entity-Relationship model—a popular high-level conceptual data model. Chapter 8 describes additional abstractions used for advanced modeling, such as generalization, specialization, and categories (union types).
Representational or implementation data models are the models used most fre-quently in traditional commercial DBMSs. These include the widely used relational data model, as well as the so-called legacy data models—the network and hierarchical models—that have been widely used in the past. Part 2 is devoted to the relational data model, and its constraints, operations and languages.5 The SQL standard for relational databases is described in Chapters 4 and 5. Representational data models represent data by using record structures and hence are sometimes called record-based data models.
We can regard the object data model as an example of a new family of higher-level implementation data models that are closer to conceptual data models. A standard for object databases called the ODMG object model has been proposed by the Object Data Management Group (ODMG). We describe the general characteristics of object databases and the object model proposed standard in Chapter 11. Object data models are also frequently utilized as high-level conceptual models, particu-larly in the software engineering domain.
Physical data models describe how data is stored as files in the computer by repre-senting information such as record formats, record orderings, and access paths. An access path is a structure that makes the search for particular database records effi-cient. We discuss physical storage techniques and access structures in Chapters 17 and 18. An index is an example of an access path that allows direct access to data using an index term or a keyword. It is similar to the index at the end of this book, except that it may be organized in a linear, hierarchical (tree-structured), or some other fashion.
2. Schemas, Instances, and Database State
In any data model, it is important to distinguish between the description of the data-base and the database itself. The description of a database is called the database schema, which is specified during database design and is not expected to change frequently. Most data models have certain conventions for displaying schemas as diagrams. A displayed schema is called a schema diagram. Figure 2.1 shows a schema diagram for the database shown in Figure 1.2; the diagram displays the structure of each record type but not the actual instances of records. We call each object in the schema—such as STUDENT or COURSE—a schema construct.
A schema diagram displays only some aspects of a schema, such as the names of record types and data items, and some types of constraints. Other aspects are not specified in the schema diagram; for example, Figure 2.1 shows neither the data type of each data item, nor the relationships among the various files. Many types of con-straints are not represented in schema diagrams. A constraint such as students majoring in computer science must take CS1310 before the end of their sophomore year is quite difficult to represent diagrammatically.
The actual data in a database may change quite frequently. For example, the data-base shown in Figure 1.2 changes every time we add a new student or enter a new grade. The data in the database at a particular moment in time is called a database state or snapshot. It is also called the current set of occurrences or instances in the
database. In a given database state, each schema construct has its own current set of instances; for example, the STUDENT construct will contain the set of individual student entities (records) as its instances. Many database states can be constructed to correspond to a particular database schema. Every time we insert or delete a record or change the value of a data item in a record, we change one state of the database into another state.
The distinction between database schema and database state is very important. When we define a new database, we specify its database schema only to the DBMS. At this point, the corresponding database state is the empty state with no data. We get the initial state of the database when the database is first populated or loaded with the initial data. From then on, every time an update operation is applied to the database, we get another database state. At any point in time, the database has a current state.8 The DBMS is partly responsible for ensuring that every state of the database is a valid state—that is, a state that satisfies the structure and constraints specified in the schema. Hence, specifying a correct schema to the DBMS is extremely important and the schema must be designed with utmost care. The DBMS stores the descriptions of the schema constructs and constraints—also called the meta-data—in the DBMS catalog so that DBMS software can refer to the schema whenever it needs to. The schema is sometimes called the intension, and a database state is called an extension of the schema.
Although, as mentioned earlier, the schema is not supposed to change frequently, it is not uncommon that changes occasionally need to be applied to the schema as the application requirements change. For example, we may decide that another data item needs to be stored for each record in a file, such as adding the Date_of_birth to the STUDENT schema in Figure 2.1. This is known as schema evolution. Most mod-ern DBMSs include some operations for schema evolution that can be applied while the database is operational.