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RDBMS Articles
Page 7 of 7
Super Key in RDBMS
Super Key is an attribute (or a set of attributes) that uniquely identify a tuple i.e. an entity in entity set.It is a superset of Candidate Key, since Candidate Keys are selected from super key.ExampleLet us see an example −Student_IDStudent_EnrollStudent_NameStudent_EmailS024545Daveddd@gmail.comS344541Jackjjj@gmail.comS224555Markmmm@gmail.comThe following are the super keys for the above table −{Student_ID}{Student_Enroll}{Student_Email}{Student_ID, Student_Enroll}{Studet_ID, Student_Name}{Student_ID, Student_Email}{Student_Name, Student_Enroll}{Student_ID, Student_Enroll, Student_Name}{Student_ID, Student_Enroll, Student_Email}{Student_ID, Student_Enroll, Student_Name, Student_Email}The following would be the candidate key from the above −{Student_ID}{Student_Enroll}{Student_Email}
Read MoreForeign Key in RDBMS
A Foreign Key creates a link between tables. It references the primary key in another table and links it.For example, the DeptID in the Employee table is a foreign key −EmpIDEmpNameEmpAgeDeptIDDeptIDDeptNameDeptZoneThe DeptID in the Department table is a Primary Key in the Department Table.The DeptID in the Employee table is a Foreign Key in the Employee Table.The below figure represents the same −Above, you can see our two tables. The Foreign Key of the Employee table is the Primary Key of the Department table.
Read MoreCandidate Key in RDBMS
Each table has only a single primary key. Each relation may have one or more candidate key. One of these candidate key is called Primary Key. Each candidate key qualifies for Primary Key. Therefore candidates for Primary Key is called Candidate Key.Candidate key can be a single column or combination of more than one column. A minimal super key is called a candidate key.ExampleEmployeeID and EmployeeEmail, both can be a Primary key; therefore both are candidate keys. Select any of the as Primary Key for your table, since a table can have only a single Primary Key.Let us see another example ...
Read MoreSecondary Key in RDBMS
What is a Secondary KeySecondary Key is the key that has not been selected to be the primary key. However, it is considered a candidate key for the primary key.Therefore, a candidate key not selected as a primary key is called secondary key. Candidate key is an attribute or set of attributes that you can consider as a Primary key.Note: Secondary Key is not a Foreign Key.ExampleLet us see an example −Student_IDStudent_EnrollStudent_NameStudent_AgeStudent_Email0969122717Manish25aaa@gmail.com0559122655Manan23abc@gmail.com0679122699Shreyas28pqr@gmail.comAbove, Student_ID, Student_Enroll and Student_Email are the candidate keys. They are considered candidate keys since they can uniquely identify the student record. Select any one of the candidate key as ...
Read MoreEntity Integrity Rule in RDBMS
For Entity Integrity Rule, each table has a Primary Key.Primary Key cannot have NULL value.Student_IDStudent_AwardsStudent_AwardsAbove, you can see our primary key is Student_ID. We cannot consider Student_Awards as the primary key since not every student would have received the award.Let us see another example −Employee_IDEmployee_NameEmployee_AgeEmployee_LocationIn the above table, the Primary Key is Employee_IDLet us now summarize the Entity Integrity Rule −Make sure that each tuple in a table is unique.Every table mush has a primary key, for example, Student_ID for a Student table.Every entity is unique.The relations Primary Key must have unique values for each row.Primary Key cannot have NULL ...
Read MoreWhat is NoSQL and is it the next big trend in databases?
What is NoSQL?As per the official Wiki definition: “A NoSQL (originally referring to “non SQL” or “non relational”) database provides a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relation databases (RDBMS). It encompasses a wide variety of different database technologies that were developed in response to a rise in the volume of data stored about users, objects and products, the frequency in which this data is accessed, and performance and processing needs. Generally, NoSQL databases are structured in a key-value pair, graph database, document-oriented or column-oriented structure.Over decades ...
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