403 Keys in Relational Databases PDF

403 Keys in Relational Databases In relational databases, keys play a crucial role in ** ** ** ** uniquely identif...

403 Keys in Relational Databases In relational databases, keys play a crucial role in ** ** ** ** uniquely identifying records within a table, establishing relationships between tables, and maintaining data integrity. *** Keys ensure that each record in a database can be accurately retrieved, updated, or deleted ** without ambiguity. * This lesson covers the different types of keys used in relational databases. ** ** ** ** Types of Keys ** ** * Primary Key * == A primary key is a unique identifier for each record in a table. ** ** == It ensures that each row in a table is distinct, meaning no duplicate values are allowed in the ~~ ~~ primary key column(s). Primary keys cannot contain null values. ** Example: In a Student table, Roll Number could be a primary key since each student ** ` ` ` ` *** *** has a unique roll number. * Candidate Key * ==* A candidate key is any attribute, or a combination of attributes, that can uniquely ** ** ** ** ** ** ** identify a row in a table. ** *== * == A table can have multiple candidate keys, but only one of them can be chosen as the ** ** ** primary key. ** == * ** Example: In the Student table, both Roll Number and Email (if each student has a ** ` ` ` ` ` ` unique email) could be candidate keys, as both can uniquely identify a student. ** ** * Alternate Key * * An alternate key is a candidate key that is not selected as the primary key. ** ** == ** ** ** ** ~~ ~~ == * It serves as an alternative way to uniquely identify records within the table. == == ** Example: If Roll Number is chosen as the primary key, Email would be an alternate ** ` ` ** ** ` ` key for the Student table. ` ` * Foreign Key * * A foreign key is an attribute in one table that links to the primary key of another table. ** ** == ** ** == * * Foreign keys establish relationships between tables, enforcing referential integrity by == ** ** ** **== ensuring that values in the foreign key column must match values in the referenced primary ** key column. ** * ** Example: In a Student table that records the student data, CourseID , which is the ** ` ` ` ` ** primary key in the Course table, could be a foreign key in the Student table. ** ` ` ** ** ` ` * Composite Key * * A composite key is a key that consists of two or more attributes used together to uniquely ** ** ** ** identify a record. *  ==* Composite keys are often used when no single attribute can uniquely identify a row. *== ** Example: In an Enrollment table, a combination of Student ID and Course ID ** ` ` ` ` ` ` might serve as a composite key, uniquely identifying each enrollment record. ** ** * Super Key (Or Superkey) * * * * * * A super key is any combination of attributes that can uniquely identify a row in a table. ** ** ==** ** == * * Super keys include all candidate keys, primary keys, and any additional attributes that == ** ** ** ** ** ** make them unique. == * ==* Super keys may contain more attributes than necessary to ensure uniqueness. *== ** Example: In the Student table, {Roll Number, Name} and {Roll Number, Email} ** ` ` ` ` ` ` are super keys because they uniquely identify each row, although Roll Number alone ** ** * ` ` could also act as a key. * A super key can include attributes that aren’t individually unique, as long as the ** ** * * combination of attributes ensures uniqueness for each row. ** ** Summary Table of Keys ** ** ~ == Key Type == ~ ~ == Purpose == ~ ~ == Example == ~ Primary Key Uniquely identifies each record Roll Number in Student in a table Candidate Key Potential attributes for unique Roll Number, Email identification Alternate Key Candidate key not chosen as Email if Roll Number is primary primary key Foreign Key Establishes a link between two Course ID in Student tables Composite Key Combines multiple attributes to Student ID and Course ID ensure uniqueness Super Key Any set of attributes that {Roll Number, Name} uniquely identifies a row