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Articles by Bhanu Priya
Page 89 of 107
When do we say the schedule is conflict equivalent(DBMS)?
Two schedules are said to be conflict equivalent if the order of any two conflicting operations are the same in both the schedules.Also, a concurrent schedule S is conflict equivalent to a serial schedule S’, if we can obtain S’ out of S by swapping the order of execution of non-conflicting instructions.Example 1Even if the schedule S1 keeps the database in the consistent state, we cannot convert it into a serial schedule and hence we conclude that the schedule is not conflict equivalent to any of the serial schedules.So, instead of considering only the read and write operation, we will ...
Read MoreWhat do you mean by schedule in DBMS?
A transaction must satisfy the ACID properties of DBMS, namelyAtomicity − Execute all or none transaction.Consistency − The DB should be consistent before and after transaction execution.Isolation − Transaction must be executed in isolated form.Durability − Roll backing should always be possible in Case of any failure.A schedule is defined as an execution sequence of transactions. A schedule maintains the order of the operation in each individual transaction. A schedule is the arrangement of transaction operations. A schedule may contain a set of transactions.We already know that a transaction is a set of operations. To run transactions concurrently, we arrange ...
Read MoreWhat is an expression tree in DBMS?
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 MoreWhat is instance and schema (DBMS)?
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 schema − It is a database design at the physical level.It is hidden below the logical schema and can be changed easily without affecting the application programs.Logical schema − It is a database design at the logical level. Programmers construct applications using logical schema.External − It is schema at view ...
Read MoreExplain the evaluation of relational algebra expression(DBMS)
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 ; C=(select max(salary) from employee where deptname=CSE); // inner block Select lastname, firstname from employee where salary>c; //outer blockWhere C represents the result returned from the inner block.The relation algebra for the inner block is Ģmax(salary) (σdname=CSE(employee))The relation algebra for the outer blocks is Πlastname, firstname(σsalary>c(employee))The query optimizer would then ...
Read MoreWhat is a query language in 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 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 MoreExplain the design constraints on the generalization and specialization (DBMS)?
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 MoreExplain join dependency in DBMS
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 MoreCreate an example table that is in its first normal form(DBMS)
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 MoreWhich normal form is the highest that satisfies the functional dependencies(DBMS)?
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+ ...
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