Types of keys in dbms
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This document defines and explains different types of keys used in relational database management systems. It describes super keys as any set of attributes that uniquely identify a record, with primary keys being a single attribute that uniquely identifies each record. Candidate keys are minimal super keys, and alternate keys are candidate keys other than the primary key. Foreign keys link two tables by referencing the primary key of one table from another table.
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What are Database keys in Database System
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What are Database Keys in Database System
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Types of keys in dbms
- 1. “TYPES OF KEYS IN DATABASE” PRESENTED BY: DARSHAN HINGU
- 2. OBJECTIVES WHAT IS KEY? Super Key Primary Key Candidate Key Alternate Key Foreign Key
- 3. WHAT IS KEY? Keys are very important part of Relational database model. To maintain data integrity (that is data should be correct and in well formed) we use concept of keys. It is used for identifying unique rows from table. They are used to establish and identify relationships between tables.
- 4. SUPER KEY A super key is any combination of columns within a table that uniquely identifies each record within that table. As an example, a table used to store customer master details could contain columns such as: Customer name Customer ID Social Security number (SSN) Address Date of birth
- 5. A certain set of columns may be extracted and guaranteed unique to each customer. Examples of super keys are as follows: Name+SSN+DOB ID+Name+SSN All keys are Super key but not vice versa.
- 6. PRIMARY KEY A primary key is a column in a table that uniquely identifies the rows in that table. The data values placed in the primary key column must be unique to each row; no duplicates can be used. Nulls are not allowed in primary key columns. For example, in the table below, CustomerNo, which displays the ID number assigned to different customers, is the primary key.
- 7. CREATE TABLE CUSTOMER ( CustomerNO int NOT NULL PRIMARY KEY, FirstName varchar(20), LastName varchar(20) ); CustomerNo FirstName LastName 1 Sally Thompson 2 Sally Henderson 3 Harry Henderson 4 Sandra Wellington
- 8. CANDIDATE KEY There can be more than one candidate keys for a table. A candidate key of a relation is a minimal super key for that relation. A candidate key can be a combination of more than one columns. A Candidate Key is a set of one or more columns that can identify a record uniquely in a table. There can be multiple Candidate Keys in one table. Candidate keys are selected from the set of super keys, the only thing we take care while selecting candidate key is: It should not have any redundant attributes.
- 9. A primary key is being selected from the group of candidate keys. The value of Candidate Key is non-null for every tuple.
- 10. ALTERNATE KEY Alternate keys are candidate keys that are not selected as primary key. Alternate key is also called “Secondary Key”. If any table have more than one candidate key, then after choosing primary key from those candidate key, rest of candidate keys are known as an alternate key of that table.
- 11. FOREIGN KEY Foreign key are those keys which is used to define relationship between two tables. Foreign key are columns that point to primary key columns in other database tables. It can accept multiple null, duplicate values. It is also known as referential integrity.
- 12. --Course Table CREATE TABLE course ( courseid NUMBER(10) NOT NULL PRIMARY KEY, --primary key CourseName varchar2(10) NOT NULL, ) ; --Student Table CREATE TABLE Student ( studentId NUMBER(10) NOT NULL PRIMARY KEY, --primary key firstName varchar2(10) NOT NULL, lastName varchar2(10) NOT NULL, courseid NUMBER(10) REFERENCES course(courseid) --foreign key ) ; Example : We can have a courseid column in the Student table which is pointing to courseid column in a course table where it a primary key.
- 16. THAN KYOU