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What is NULL? Give an example to described testing for NULL in SQL?
The NULL SQL keyword is used to represent either a missing value or a value which is not applicable in a relational table. Consider there is a relation:
Person(id, name, address, phone)Now to search ids and names of person who do not have a phone is: Select id, name from Person where phone is null
Objectives After reading this unit you should be able to: Explain storage of databases on hard disks; Discuss the execution of various file Organisation techniques;
Define Shared locks? Ans: Shared lock: Throughout concurrent execution of transactions, earlier than a transaction can access a data item, it has to obtain a lock on it. Now he
What is a guard condition? Explain it with a suitable example. The guard-condition is a Boolean expression written with respect to parameters of triggering event and attributes
What is inheritance? Inheritance is an object orientation concept that allows reusability of code / design. Basic sense of inheritance is that if one class is predefined than a
Super keys of a relation can have extra attributes. Candidate keys are minimal super key, i.e. such a key have no extraneous attribute. An attribute is known as extraneous if even
Relational and E-R Model There are three types of constraints on relational database that have: DOMAIN CONSTRAINT PRIMARY KEY CONSTRAINT INTEGRITY CONSTRAINT
Define in which normalisation form if an attribute of a composite key is dependent on an attribute of other composite key. Ans: 3NF A relation is in 3 NF if an attribute of
Evaluate: a.When transactions T1 and T2 are in a deadlock, one of the transactions will eventually implement to completion and let the other one continue implementation. b.
Database Errors: An error is said to have occurred if the implementation of a command to manipulate the database cannot be successfully finished either due to inconsistent data or
WHERE and HAVING clause in SQL ? The WHERE clause is commonly used for implementing conditions on each tuple of the relation. The HAVING clause is used in combination along wi
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