A transaction is a unit of database processing which contains a set of operations. For example, deposit of money, balance enquiry, reservation of tickets etc.Every transaction starts with delimiters begin transaction and terminates with end transaction delimiters. The set of operations within these two delimiters constitute one transaction.main() {   ... Read More

Cost-based optimization is expensive. Heuristics are used to reduce the number of choices that must be made in a cost-based approach.RulesHeuristic optimization transforms the expression-tree by using a set of rules which improve the performance. These rules are as follows −Perform the SELECTION process foremost in the query. This should ... Read More

Expression tree is also called a query tree. It demonstrates an evaluation plan.ExampleConsider an evaluation plan for the given query −Select name from customer, account where customer.name=account.name and account.balance>2000;There are two evaluation plans −Πcustomer.name(σcustomer.name=account.name^ account.balance>2000(customerXaccount)Πcustomer.name(σcustomer.name=account.name(customerXσ account.balance>2000(account)The expression trees for the above evaluation plans are as follows −Cost evaluator evaluates the ... Read More

The overall design of the database is called database schema. Schema will not be changed frequently. It is the logical structure of a database. It does not show the data in the database.The schema is pictorially represented as follows −Types of SchemaThe different types of schemas are as follows −Physical ... Read More

SQL queries are decomposed into query blocks. One query block contains a single SELECT-FROM-WHERE expression, as well as GROUP BY and HAVING clause (if any). Nested queries are split into separate query blocks.ExampleConsider an example given below −Select lastname, firstname from employee where salary>(select max(salary) from employee where deptname =CSE ... Read More

Query is a question or requesting information. Query language is a language which is used to retrieve information from a database.Query language is divided into two types as follows −Procedural languageNon-procedural languageProcedural languageInformation is retrieved from the database by specifying the sequence of operations to be performed.For Example: Relational algebraStructure ... Read More

The different types which we need to consider while designing generalization and specialization in the Database Management System (DBMS) are as follows −Conditional definitionAttribute definedUser definedDisjoint definedOverlapping constraintCompleteness constraintLet us now understand them one by one.Conditional definitionCreate one database, and keep conditions on one attribute for example attendance. This type ... Read More

Join dependency is a constraint which is similar to functional dependency or multivalued dependency. It is satisfied if and only if the relation concerned is the join of a certain number of projections. Such type of constraint is called join dependency.Let's consider a special class of join dependencies which help ... Read More

To understand why database management systems are necessary, let's look at a typical "file processing system" supported by conventional operating systems.ExampleTake an application in a saving bank. Saving accounts and customer records are kept in permanent system files.Application programs are written to manipulate files to perform some tasks like follows ... Read More

If a table has data redundancy and is not properly normalized then it will be difficult to handle and update. If a relation contains composite or multivalued attributes it violates the first normal form.A table is in 1Nf if the following conditions exist −There are only single valued attributes.Attribute domain ... Read More