Found 338 Articles for DBMS

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 −Procedural languageNon-procedural languageProcedural languageInformation is retrieved from the database by specifying the sequence of operations to be performed.For Example − Relational algebra.Structure Query language (SQL) is based on relational algebra.Relational algebra consists of a set of operations that take one or two relations as an input and produces a new relation as output.Types of Relational Algebra operationsThe different types of relational algebra operations are as follows −Select operationProject operationRename operationUnion operationIntersection operationDifference operationCartesian ... 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 Query language (SQL) is based on relational algebra.Relational algebra consists of a set of operations that take one or two relations as an input and produces a new relation as output.The different types of relational algebra operations are −Select operationProject operationRename operationUnion operationIntersection operationDifference operationCartesian product operationJoin operationDivision operation.Select, project, ... 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 of constraint is defined on a single attribute which is further dividing an entity into two sub entity sets which will give information for the given attribute.ExampleGiven below is an example of the conditional definition database −Attribute definedThis refers to specifying conditions on more than one attribute.ExampleConsider a database for ... 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 cost of different evaluation plans and chooses the evaluation plan with lowest cost. Disk access time, CPU time, number of operations, number of tuples, size of tuples are considered for cost calculations.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 to capture data dependencies present in a hierarchical data structure.Example 1The above hierarchical organization informs regarding ward and patients currently admitted to a ward depend only on the hospital but not the facilities present in that hospital. Since hospitals have multiple wards, functional dependencies are not adequate to describe the ... 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 −Debit or credit an account.Add a new account.Find an account balance.Generate monthly statements.Development of the system proceeds as new application programs must be written as the need arises, new permanent files are created as required, but over a long period of time files may be in different formats and application ... 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 does not change.There is a unique name for every attribute or column.The order in which data is stored does not matter.ExampleConsider the table given below −IdNameCourse1AC1, C22EC33BC2, C3The above table is in un normalized form, the course attribute is having multivalued attributes which violates 1NF.Now we represent the above table ... Read More

Let's take the dependencies F= {AB->CDEF, AF->ED, F->B} which ONE of the following is the highest normal form that a table R=ABCDEF could satisfy.Select any one from the following −No normal form would be satisfied.Boyce-Codd Normal Form1st Normal Form2nd Normal Form3rd Normal FormGiven Functional dependencies of F are as follows −AB ---> CDEFAF ---> EDF ---> BNow in R.H.S (Right Hand Side) of Functional dependencies of F, attributes B, C, D, E, F are all present but attribute A is missing. So, attribute A must be the part of Super Key.We have to find the closure of A (A+), A+ ... Read More

In the database management system there are several rules to manage the tables and do the changes like insertion, deletion, updatation and there are different stages at which the tables are organized and these stages are known as normal forms.It is a standard structure of relational databases in which a relation may be inserted in another relation.Five normal formsThere are five normal forms which are explained below −First normal formThe first normal form defines the definition of the table. The rules of the first normal form are as follows −Each column must have a single value.Each column has a unique ... Read More

Multivalued dependency (MVD) is having the presence of one or more rows in a table. It implies the presence of one or more other rows in that same table. A multivalued dependency prevents fourth normal form. A multivalued dependency involves at least three attributes of a table.It is represented with a symbol "->->" in DBMS.X->Y relates one value of X to one value of Y.X->->Y (read as X multidetermines Y) relates one value of X to many values of Y.A Nontrivial MVD occurs when X->->Y and X->->z where Y and Z are not dependent are independent to each other. Non-trivial ... Read More