2.1.2 Schemas, Instances, and Database StateIn 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 databaseschema, which is specified during database design and is not expected to changefrequently.6 Most data models have certain conventions for displaying schemas asdiagrams. 7 A displayed schema is called a schema diagram. Figure 2.1 shows aschema diagram for the database shown in Figure 1.2; the diagram displays thestructure of each record type but not the actual instances of records. We call eachobject 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 ofrecord types and data items, and some types of constraints. Other aspects are notspecified in the schema diagram; for example, Figure 2.1 shows neither the data typeof 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 studentsmajoring in computer science must take CS1310 before the end of their sophomore yearis 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 newgrade. The data in the database at a particular moment in time is called a databasestate or snapshot. It is also called the current set of occurrences or instances in theSection_identifier SemesterCourse_number InstructorYearSECTIONCourse_name Course_number Credit_hours DepartmentCOURSEName Student_number Class MajorSTUDENTCourse_number Prerequisite_numberPREREQUISITEStudent_number GradeSection_identifierGRADE_REPORTFigure 2.1Schema diagram for thedatabase in Figure 1.2.6Schema changes are usually needed as the requirements of the database applications change. Newerdatabase systems include operations for allowing schema changes, although the schema changeprocess is more involved than simple database updates.7It is customary in database parlance to useschemas as the plural forschema, even thoughschemata isthe proper plural form. The wordscheme is also sometimes used to refer to a schema.2.2 Three-Schema Architecture and Data Independence 33database. In a given database state, each schema construct has its own current set ofinstances; for example, the STUDENT construct will contain the set of individualstudent entities (records) as its instances. Many database states can be constructedto correspond to a particular database schema. Every time we insert or delete arecord or change the value of a data item in a record, we change one state of thedatabase 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. Weget the initial state of the database when the database is first populated or loadedwith the initial data. From then on, every time an update operation is applied to thedatabase, we get another database state. At any point in time, the database has acurrent state. 8 The DBMS is partly responsible for ensuring that every state of thedatabase is a valid state—that is, a state that satisfies the structure and constraintsspecified in the schema. Hence, specifying a correct schema to the DBMS isextremely important and the schema must be designed with utmost care. TheDBMS stores the descriptions of the schema constructs and constraints—also calledthe meta-data—in the DBMS catalog so that DBMS software can refer to theschema whenever it needs to. The schema is sometimes called the intension, and adatabase state is called an extension of the schema.Although, as mentioned earlier, the schema is not supposed to change frequently, itis not uncommon that changes occasionally need to be applied to the schema as theapplication requirements change. For example, we may decide that another dataitem needs to be stored for each record in a file, such as adding the Date_of_birth tothe 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 whilethe database is operational.2.2 Three-Schema Architectureand Data IndependenceThree of the four important characteristics of the database approach, listed inSection 1.3, are (1) use of a catalog to store the database description (schema) so asto make it self-describing, (2) insulation of programs and data (program-data andprogram-operation independence), and (3) support of multiple user views. In thissection we specify an architecture for database systems, called the three-schemaarchitecture,9 that was proposed to help achieve and visualize these characteristics.Then we discuss the concept of data independence further.8The current state is also called thecurrent snapshot of the database. It has also been called adatabaseinstance, but we prefer to use the terminstance to refer to individual records.9This is also known as the ANSI/SPARC architecture, after the committee that proposed it (Tsichritzis