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DBMS Training & SQL Tutorial : Data Models

apid Theory you need to know!
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  • A Data Model is the methodology used by a particular DBMS to organize and access the data

  • Hierarchical, Network and Relational Model are the three popular data models. However, the relational model is more widely used.

Hierarchical Model
  • The hierarchical model was developed by IBM in 1968.

  • The data is organize in a tree structure where the nodes represent the records and the branches of the tree represent the fields.

  • Since the data is organized in a tree structure, the parent node has the links to its child nodes.

  • If we want to search a record, we have to traverse the tree from the root through all its parent nodes to reach the specific record. Thus, searching for a record is very time consuming.

  • The hashing function is used to locate the root.

  • SYSTEM2000 is an example of hierarchical database.

Network Model
  • Record relationship in the network model is implemented by using pointers.

  • Record relationship implementation is very complex since pointers are used. It supports many-to-many relationships and simplified searching of records since a record has many access paths.

  • DBTG Codasyl was the first network database.

Relational Model
  • The Relational Model, organizes data in the form of independent tables (consisting of rows and columns) that are related to each other.

  • A table consists of a number of rows (records/tuples) and columns (attributes). Each record contains values for the attributes.

  • The degree of the table denotes the number of columns.

  • A domain in the relational model is said to be atomic is it consists of indivisible units. For example, name is not atomic since it can be divided into first name and last name.

  • E. F. Codd laid down 12 rules (known as Codd's 12 rules) that outline the minimum functionality of a RDBMS. A RDBMS must comply with at least 6 of the rules.

  • A Super Key is a of attributes that collectively identify a entity in a entity set. For example, the bank account number is a super key in the bank accounts table.

  • A Candidate Key (also known as Primary Key) is the smallest subset of the super key for which there does not exist a proper subset that is a super key.

  • Out of the multiple candidate keys, only one is selected to be the primary key and the remaining are alternate keys.

  • A foreign key is the primary key of a table that is placed into a related table to represent one-to-many relationship among these tables.


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