HIERARCHICAL DATABASE

¡Fields or records structured in nodes

¡Viewed as branches of an upside-down tree

¡Each item is subordinate to its parent node

¡Only one parent per node

¡The subordinate item is the child node to the parent

¡If parent node is deleted, all the child nodes are as well deleted

¡New parent node must be created before adding a new child node

¡No direct relationships between child nodes

NETWORK DATABASE

¡Also has hierarchical node arrangement

¡But here child nodes may have more than one parent node, or a many-to-many relationship

¡The interconnected design allows for access via multiple pathways

RELATIONAL DATABASE

¡No pre-determined access paths

¡Data stored in a collection of columns and rows called a table, or a relation

¡Tables may be electronically linked via a key field containing common data

¡Easy to add, delete and modify the data and the table structures

Relational Terminologies 

¡Table or Relation

¡Null values

¡Duplicate Values

¡Changeable Values

¡Primary Keys

¡Foreign Keys

TABLE OR RELATION

¡Table will store information for a particular entity

¡Table name must be unique

¡ The Table name should be descriptive

¡Column Name must be unique within the Table

¡Rows must be unique

NULL VALUES

¡missing or unknown value in a column of a table

¡Nulls are not the same as zeros

¡Most arithmetic operations can be performed on zero values

¡nulls must be excluded from mathematical manipulations

DUPLICATE VALUES

¡A duplicate value is a value in a column of a table that exactly matches some other values within the same column.

CHANGEABLE VALUES

¡value in a table that may vary over time.

¡Most values in most tables are Changeable

¡You can prevent changes when it is desirable to prevent changes in a given column of a table

PRIMARY KEYS

¡Uniquely identify each row of that table.

¡ Every table must have only one Primary key

RULES FOR PRIMARY KEY

¡Must always have a value (null values are not allowed)

¡Value should be unique (duplicate values are not allowed)

¡Value should not change over time   

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