DATA BASE KEYS.docx
- 1. DATA BASE KEYS A key is an attribute (or set of attributes) that uniquely identifies any record (from tuple). EMPLOYEE (referencing/child/foreign/related) {empid,name,adharno,email_id,deptid} Emp_id NAME ADHAR NO Email id Dept_id 12345 a 11 ab 1 2 a 22 bc 1 3 b 33 cd null Department(referenced/parent/primary/master) Dept id loc 1 del 2 kol 3 mumbai Super keys Emp id, Adharnumber Email id Emp id, adharnumber,emailid Adharno , email id Emp_id ,adharnumber Empid , name , departmentid Empid,name,adharno,emailed,deptid
- 2. Purpose Key is used to uniquelyidentify any records or row of data from the table. It is used to establish and identify relationshipbetween table. Super key- A super key is a combinationof all possible attributes that can uniquelyidentifythe rows /tuples in the given relation. Super key is a superset of candidatekeys. A table can have multiple super keys. A SK may have additionalattributethat are not needed for uniqueidentity . All possible combinationsof attributes. Candidatekey- Candidatekey is a minimalsuperkey or a super key with no redundantattributes. It is called a minimal superkey because we select a candidate key from a set of super key such that selected candidatekey is the minimum attribute required to uniquelyidentify the table. Candidatekeys are defined as distinct set of attributes from which primary key can be selected. Candidatekeys are not allowedto have null values. Primary key – choose a key from candidatekey A primary key is one of the candidatekey chosen by database designer to uniquelyidentify the tuple in the relation.
- 3. The value of pk can never be null The value of pk must alwaysbe unqiue. The values of pk can never be changed – no updationis possible The value of pk must be assigned when inserting a record A relation is allowed to have one primary key. Alternate key – out of all candidatekeys only one is selected as pk , remaining keys are called alternate keys Empid- primary key Name, adharno,email id, dept id- alternate keys. Foreign key – A key used to link two tables together An attribute / set of attribtes in one table that referes to pk in another table. The purpose – to ensure referential integrity of the data. Foreign key references the pk of the table. Fk can only take pk Fk can be null. There is no restriction that foreign key is unique. Composite key / compound key – a key that has more than one attribute . Cust_id Order_id Productcode Product
- 4. count CO1 001 P111 5 CO2 012 P111 8 Co3 012 P222 6 Co1 001 P333 9 {Custid, productcode} One to many Relational Query Language – query language is the language in which user requests informationfrom databse. e.g sql – structured query language query – retrieval 2types of query languages Procedural query language –the user instructs the system to perform a series of operationsto produce the desired results. User tellswhat data to be retrieved from data base and how to retrieve. Non prodecural query language – (declarative) User instructs the system to produce the desired results without telling step by step process. User tellswhat data to be retrieved from data base but doesn’t tell how to retrieve it . Two pure(mathematical)query language
- 5. Relationalgebra – procedural query language It is more operational Very useful for executionalplan. Relationalcalculus Non procedural query language It is non operational Tuple relationlcalculus Domain relationalcalculus Relationalmodel RDBMS RELATIONAL algebra,calculus sql algorithm code concept reality theoretical practical