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Relational Model: The relational model uses a collection of tables to represent both data and the relationships among those data. Each table has multiple columns and each column has a unique name. Figure presents a sample relational database comprising three tables: One shows details of bank customer, the second shows accounts belong to which customers. The relational is an example of a record based model. Record based models are so named because the database is structured in fixed format records of several types. Each table contains of a particular types. Each types defines a fixed number of fields or attributes. The relational data model is the most widely used data model, and a vast majority of current database systems are based on the relational model. The relational model is at a lower level of abstractions than the E-R model.
Other Data Models: The object- oriented data model is another data model that has seen increasing attention. The object- oriented model can be seen as extending the E-R model with notions of encapsulation, methods, and object identity. Historically, two other data models, the network data model and the hierarchical data model, preceded the relation of data model. These models were tied closely to the underlying implementation, and complicated the task of modeling data. Semi structured data models permit the specification of data where indivisible data items of the same type may have different sets of attributes. This is in contrast with the data models mentioned earlier, where every data item of a particular type must have the same set of attributes. The extensible markup language (XML) is widely used to represent semi- structured data.
Keys:
Superkey: It is a set of one or more attributes that taken collectively; allow us to identify an entity set. Candidate key: Superkeys from which no proper subset is a superkey. Such minimal superkeys are called candedate keys.
Primary key: Allows us to distinguish between different entities of set. It must be chosen s.t. its attributes are never or rarely changed.
Foreign Key: When relation schema say r1may include among its attributes the primary key of another relation schema say r2. This attribute is called foreign key from r1, referencing r2, we sometimes define a primary key and sometimes not but there also candidate key will exist. Two attributes taken together can make a key called as composite primary key.
Using Literal Character String Example: To print IS A as one string with column name employee SELECT ENAME || ' IS A ' || JOB AS "EMPLOYEE" FROM EMP;
Describe in brief about DBMS ?
Indexed (Indexed Sequential) File Organisation It organises the file like a big dictionary, i.e., records are kept in order of the key but an index is stored which also allows
Eliminate Redundant Data In the "Database Relation" above, the primary key is made up of the D-ID and the Emp-ID. The database-name relies only on the D-ID. The similar databas
Parallelism of transaction execution: A transaction can be divided into various sub-queries that can operate on fragments in parallel. This enhances the degree of concurrency in t
Scenario 1: Power goes out after the first instruction. What happens?
Aggregation Aggregation is stronger form of association. It represents the "has-a" or "part-of" relationship. An aggregation association depicts a complex object which is comp
Consider employee table of (i) and list names of department(s) for which average salary for department is more than 10,000. (SELECT d_name FROM dept WHERE d_code IN (SELECT d_c
how draw er diagram
What is a primary key? Primary key is chosen by the database designer as the principal means of identifying an entity in the entity set.
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