key-database management
TRANSCRIPT
KeyDBMS
Introduction
Keys(Name clearly define)
A key part of a relational database and a vital part of the structure of a table. They ensure each record within a table can be uniquely identified by one or a combination of fields within the table. They help enforce integrity and help identify the relationship between tables.
Keys
Super Key
Candidate Key
Primary Key
Super Key
A Super key is any combination of fields within a table that uniquely identifies each record within that table.
BookBook ID Name Author
B1 Xyz A1
B2 ABC A1
B3 XYZ A2
B4 PQR A3
B5 RST A1
B6 ABC A3
Candidate Key
Candidate key is a subset of super key
A candidate key is a single field or the least combination of fields that uniquely identifies each record in table
The least combination of fields distinguishes a candidate key from a super key.
Every table must have at least one candidate
Properties Unique
Not null
Contains minimum number of fields to ensure uniqueness
Must uniquely identify each record in the table
Book ID Name Author
B1 XYZ A!
B2 ABC A1
B3 XYZ A2
B4 PQR A3
B5 RST A1
B6 ABC A3
Name BookId Author Name Author
Primary Key
It is a candidate key
Uniquely identify a specific instance of an entity
Primary key cannot contain any Null value because we cannot uniquely identify multiple Null values.
Properties Stable
Minimal
Definitive
Accessible
Book ID Name Author
B1 Xyz A!
B2 ABC A1
B3 XYZ A2
B4 PQR A3
B5 RST A1
B6 ABC A3
Book ID Name Author
CREATE TABLE Persons(
P_Id int NOT NULL PRIMARY KEY,LastName varchar(255) NOT NULL,FirstName varchar(255),Address varchar(255),City varchar(255)
);
CREATE TABLE Person
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
);
Foreign Key
Foreign key is a field in a relational table that matches the primary key column of another table.
It can be used to link two tables together
CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
);
CREATE TABLE products
( product_id numeric(10) not null,
supplier_id numeric(10) not null,
CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id)
);
Remaining
Alternate Key
Remaining candidate key which are not selected as primary key
Composite Key
Primary key made up of more than one attribute