aws Flashcards

Question

Cardinality(tables)

Answer 1

no of tuples

Answer 2

conditions which must hold for data present in database called Constraints. These constraints are checked before performing any operation (insertion, deletion and updation) in database. If there is a violation in any of constraints, operation will fail A). Domain Constraints:(where ) An attribute can only take values which lie inside the domain range. eg: If a constraint AGE>0 is applied on STUDENT relation, inserting negative value of AGE will result in failure B). Key Integrity:(primary key) Every relation in the database should have atleast one set of attributes which defines a tuple uniquely. Those set of attributes is called key. e.g: ROLL_NO in STUDENT is a key. No two students can have same roll number. So a key has two properties: * It should be unique for all tuples. * It can’t have NULL values. C). Referential Integrity: When one attribute of a relation can only take values from other attribute of same relation or any other relation, it is called referential integrity. Let us suppose we have 2 relations.

Answer 3

Relational Algebra is procedural query language, which takes Relation as input and generate relation as output. Relational algebra mainly provides theoretical foundation for relational databases and SQL.

Answer 4

1). Projection (π) Projection is used to project required column data from a relation. 2). Selection (σ) Selection is used to select required tuples of the relations σ (c>3)R π (σ (c>3)R ) w 3). Union (U)(no dupes) Union operation in relational algebra is same as union operation in set theory Both relation must have same set of Attributes. R u S ∏ CUSTOMER_NAME(BORROW) ∪ ∏ CUSTOMER_NAME(DEPOSITOR) 4).Intersection (∩)(dupes) Suppose there are two tuples R and S. The set intersection operation contains all tuples that are in both R & S. 5). Rename (ρ) Rename is a unary operation used for renaming attributes of a relation. ρ (a/b)R will rename the attribute ‘b’ of relation by ‘a’. 6). Cross Product ( X ) ( each row in a with each row in b) Cross product between two relations , lets say A and B, so cross product, A X B will results all the attributes of A followed by each attribute of B. Each record of A will pairs with every record of B. 7).Natural Join ( ) ⋈(coomon attribute) Natural join is a binary operator. Natural join between two or more relations will result set of all combination of tuples where they have equal common attribute. 8).Conditional Join Conditional join works similar to natural join. In natural join, by default condition is ‘equal’ between common attribute. While in conditional join, we can specify any condition such as greater than, less than, not equal 9).Set Difference (-) Set Difference in relational algebra is same set difference operation as in set theory. Suppose there are two tuples R and S. The set intersection operation contains all tuples that are in R but not in S. 10). Divide ( ÷) Division operator A÷B can be applied if and only if: * Attributes of B is proper subset of Attributes of A. * The relation returned by division operator will have attributes = (All attributes of A – All Attributes of B) * The relation returned by division operator will return those tuples from relation A which are associated to every B’s tuple

Answer 5

SQL is a standard language for storing, manipulating and retrieving data in databases. SQL is used to communicate with a database.

Answer 6

1).To create a database CREATE DATABASE Database_name ; 2).To list the existing databases SHOW DATABASES; 3).To use an already existing database USE Database_name; eg: USE COLLEGE; 4).To create a table CREATE TABLE Table_name(column1 datatype, column2 datatype...., columnn datatype); eg:CREATE TABLE Student(Name VARCHAR(20), Rollno INT, Dob DATE, Department VARCHAR(20)); 5).To insert values to a table INSERT INTO Table_name VALUES(Value1,Value2....Valuen); eg:INSERT INTO Student VALUES(‘Asha’,1, ‘1995-07-01’, ‘CS’); 6).SELECT a).To select certain columns of all the entries from table SELECT Column_a, Column_b FROM Table_name; eg: SELECT Rollno,Name FROM Student; b).To select all the rows SELECT * FROM Table_name 7).To select distinct elements of a column SELECT DISTINCT Column_a FROM Table_name; eg: SELECT DISTINCT Name FROM Student; 8).OR To display the record of those students whose rollno is 1 or 2, eg: SELECT *FROM Student WHERE Rollno=1 OR Rollno=2 9).AND To display the record of those students whose rollno is 1 and name is Asha, eg: SELECT * FROM Student WHERE Rollno=1 AND Name=’Asha’; 10).NOT To display the records of those students whose name is not ‘akhil’, eg: SELECT * FROM Student WHERE NOT Name=’Akhil’; 11).ALTER a). To add a new column in a table ALTER TABLE Table_name ADD Column_name datatype; eg: ALTER TABLE Student ADD Email VARCHAR(20); b).To delete a column from a table ALTER TABLE Table_name DROP COLUMN Column_name; eg: ALTER TABLE Student DROP COLUMN Email; c).To change the datatype of a column in a table ALTER TABLE Table_name ALTER COLUMN Column_name datatype; eg: ALTER TABLE Student ALTER COLUMN Rollno VARCHAR(20); 12).DROP To drop an existing table from db.This will result in loss of complete info stored in table and schema. DROP TABLE Table_name; eg:DROP TABLE Student; 13).DELETE To delete tuples from a table a). DELETE FROM Table_name WHERE Condition; eg: DELETE FROM Student WHERE Rollno=5; b).To Delete entire records in table DELETE FROM Table_name; 14). UPDATE To modify the existing records in a table UPDATE Table_name SET Column1=value1....Columnn=valuen WHERE Condition; Eg: UPDATE Student SET Name=’Anjali’ WHERE Rollno=1; 15). IN Same as OR SELECT Column_names FROM Table_name WHERE Column_name IN (Value1,Value2..); eg: SELECT *FROM Student WHERE Rollno IN (1,3,5); 16). ORDER BY To sort the result in asc/desc order. Ascending by default. SELECT Column1,Column2,.. FROM Table_name ORDER BY Column1,Column2...ASC/DESC; eg: To sort the entire table according to the marks obtained in DBMS subject, SELECT * FROM Student ORDER BY dbms_Marks; 22). LIKE To search for a specific pattern in a column SELECT Column_names FROM Table_name WHERE Columnn LIKE Pattern; a). find any value starting with ‘a’ SELECT Rollno,Name FROM Student WHERE Name LIKE ‘a%’; b). find any value that have ‘a’ in 2nd position SELECT Rollno,Name FROM Student WHERE Name LIKE ‘_a%’; 23). BETWEEN : To select values within a range(begin and end values are included). SELECT Column_names FROM Table_names WHERE Column_name BETWEEN Value1 AND Value2; eg : SELECT Name FROM Student WHERE Rollno BETWEEN 1 AND 5; 24). GROUP BY : To group the rows that have same values into summary rows. It is often used along with aggregate functions. SELECT Columnnames FROM Table_name WHERE Condition GROUP BY Columnnames ORDER BY Columnnames; eg1: To find the number of customers in each country, SELECT COUNT(CustomerId), Country FROM Customers GROUP BY Country; eg2: To sort the country names according to the number of customers in each country , SELECT COUNT(CustomerId), Country FROM Customers GROUP BY Country ORDER BY COUNT(CustomerId); 25). HAVING : The HAVING clause was added to SQL because the WHERE keyword cannot be used with aggregate functions. SELECT column_name(s) FROM table_name WHERE condition GROUP BY column_name(s) HAVING condition ORDER BY column_name(s);

Answer 7

17). COUNT To get the count of values in a column a).SELECT COUNT(Columnn) FROM Table_name; eg: SELECT COUNT(Rollno) FROM Student; b).To get the number of rows that matches a condition SELECT COUNT(Columnn) FROM Table_name WHERE Condition; 18). MIN() To get the smallest value in a column SELECT MIN(Column_name) FROM Table_name; 19).MAX() : To get the largest value in a column SELECT MAX(Column_name) FROM Table_name; 20). SUM() : Returns the total of a numeric column SELECT SUM(Column_name) FROM Table_name; 21). AVG( ) Returns the average of a numeric column. SELECT SUM(Column_name) FROM Table_name;

Answer 8

Used to comblne rows from two or more tables based on a related column between them a).INNER JOIN SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column_name = table2.column_name; b).LEFT JOIN The LEFT JOIN keyword returns all records from the left table (table1), and the matching records from the right table (table2). Syntax : SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name; c).RIGHT JOIN The RIGHT JOIN keyword returns all records from the right table (table2), and the matching records from the left table (table1). The result is 0 records from the left side, if there is no match. Syntax : SELECT column_name(s) FROM table1 RIGHT JOIN table2 ON table1.column_name = table2.column_name; d)FULL JOIN The FULL OUTER JOIN keyword returns all records when there is a match in left (table1) or right (table2) table records. Syntax: SELECT column_name(s) FROM table1 FULL OUTER JOIN table2 ON table1.column_name = table2.column_name WHERE condition;

Answer 9

In nested queries, a query is written inside a query. The result of inner query is used in execution of outer query. SELECT column_names FROM tablenames WHERE column_name OPERATOR (SELECT column_name FROM tablename WHERE condition)

Answer 10

* In SQL, a view is a virtual table, based on the result-set of an SQL statement A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.ie,We can create a view by selecting fields from one or more tables present in the database. A View can either have all the rows of a table or specific rows based on certain condition..A View can be created from a single table or multiple tables. single table CREATE VIEW DetailsView AS SELECT NAME, ADDRESS FROM StudentDetails WHERE S_ID < 5; multiple tables CREATE VIEW MarksView AS SELECT StudentDetails.NAME, StudentDetails.ADDRESS, StudentMarks.MARKS FROM StudentDetails, StudentMarks WHERE StudentDetails.NAME = StudentMarks.NAME; Drop the view : We can delete or drop a View using the DROP statement. Syntax: DROP VIEW view_name; * For example, if we want to delete the View MarksView, we can do this as: DROP VIEW MarksView;

Answer 11

1. Restricting data access – Views provide an additional level of table security by restricting access to a predetermined set of rows and columns of a table. 2. Hiding data complexity – A view can hide the complexity that exists in a multiple table join. 3. Simplify commands for the user – Views allows the user to select information from multiple tables without requiring the users to actually know how to perform a join. 4. Store complex queries – Views can be used to store complex queries. 5. Rename Columns – Views can also be used to rename the columns without affecting the base tables provided the number of columns in view must match the number of columns specified in select statement. Thus, renaming helps to to hide the names of the columns of the base tables. 6. Multiple view facility – Different views can be created on the same table for different users.

Answer 12

A stored procedure is a prepared SQL code that you can save, so the code can be reused over and over again. So if you have an SQL query that you write over and over again, save it as a stored procedure, and then just call it to execute it. You can also pass parameters to a stored procedure, so that the stored procedure can act based on the parameter value(s) that is passed. * Instead of inserting them in the code of each application, they may be put in a stored procedure and reused. Syntax DELIMITER $$ CREATE PROCEDURE Procedure_name( Parameter_list ) BEGIN Statements; END $$ * To invoke the procedure CALL Procedure_name(argument_list); * To drop a procedure DROP PROCEDURE Procedure_name; eg DELIMITER $$ CREATE PROCEDURE GetCustomers() BEGIN SELECT customerName,country FROM customers ORDER BY customerName; END $$ CALL GetCustomers(); * The first time you invoke a stored procedure, MySQL looks up for the name in the database catalog, compiles the stored procedure’s code, place it in a memory area known as a cache, and execute the stored procedure. STORED PROCEDURES WITH SINGLE PARAMETER PASSING To Create a procedure that find the number of students who comes from a particular place, DELIMITER $$ CREATE PROCEDURE Findcount( IN Place VARCHAR(20)) BEGIN SELECT COUNT(*) FROM STUDENT WHERE City=Place; END $$ To invoke the procedure CALL Findcount(‘kannur’); $$ STORED PROCEDURES WITH MULTIPLE PARAMETER PASSING To Create a procedure that find the number of students of a particular gender who comes from a particular place, DELIMITER $$ CREATE PROCEDURE Gettotal( IN Place VARCHAR(20), IN Gend VARCHAR(10)) BEGIN SELECT COUNT(*) FROM STUDENT WHERE City=Place AND Gender=Gend; END $$ To invoke the procedure, CALL Gettotal(‘Kannur’,’female’); $$ STORED PROCEDURES WITH IF-THEN-ELSE STATEMENT DELIMITER $$ CREATE PROCEDURE Getstatus( IN Registerno INT, OUT Status VARCHAR(20)) BEGIN DECLARE mark INT DEFAULT 0; SELECT dbms INTO mark FROM STUDENT WHERE Regno=Registerno; IF mark >20 THEN SET Status=’pass’; else SET Status=’fail’; END IF; END $$ To invoke the procedure, CALL Getstatus(9,@Status); SELECT @Status; $$

Answer 13

Stored procedures provide some crucial benefits, which are: * Reusable: Multiple users and applications can easily use and reuse stored procedures by merely calling it. * Easy to modify: You can quickly change the statements in a stored procedure as and when you want to, with the help of the ALTER TABLE command. * Security: Stored procedures allow you to enhance the security of an application or a database by restricting the users from direct access to the table. * Low network traffic: The server only passes the procedure name instead of the whole query, reducing network traffic. * Increases performance: Upon the first use, a plan for the stored procedure is created and stored in the buffer pool for quick execution for the next time.

Answer 14

A trigger is a stored procedure in database which automatically invokes whenever a special event in the database occurs. For example, a trigger can be invoked when a row is inserted into a specified table or when certain table columns are being updated. Syntax: create trigger [trigger_name] [before | after] {insert | update | delete} on [table_name] [for each row] [trigger_body] create trigger [trigger_name]: Creates or replaces an existing trigger with the trigger_name. 1. [before | after]: This specifies when the trigger will be executed. 2. {insert | update | delete}: This specifies the event (DML operation). 3. on [table_name]: This specifies the name of the table associated with the trigger. 4. [for each row]: This specifies a row-level trigger, i.e., the trigger will be executed for each row being affected. 5. [trigger_body]: This provides the operation to be performed as trigger is fired. BEFORE and AFTER of Trigger: BEFORE triggers run the trigger action before the triggering statement is run. AFTER triggers run the trigger action after the triggering statement is run. example create trigger stud_marks before INSERT on Student for each row set new.total =new.subj1 + new.subj2 + new.subj3, new.average = new.total /3 ;

Answer 15

* An exception is an error which disrupts the normal flow of program instructions. * An Exception is an error situation, which arises during program execution. * PL/SQL provides us the exception block which raises the exception thus helping the programmer to find out the fault and resolve it. * When an error occurs & exception is raised, normal execution is stopped and control transfers to exception handling part. ACCESS_INTO_NULL It is raised when a NULL object is automatically assigned a value. CASE_NOT_FOUND It is raised when none of the choices in the "WHEN" clauses of a CASE statement is selected, and there is no else clause. COLLECTION_IS_NULL It is raised when a program attempts to apply collection methods other than exists to an uninitialized nested table or varray, or the program attempts to assign values to the elements of an uninitialized nested table or array. DUP_VAL_ON_INDEX It is raised when duplicate values are attempted to be stored in a column with unique index. INVALID_CURSOR It is raised when attempts are made to make a cursor operation that is not allowed, such as closing an unopened cursor. INVALID_NUMBER It is raised when the conversion of a character string into a number fails because the string does not represent a valid number. LOGIN_DENIED It is raised when s program attempts to log on to the database with an invalid username or password. NO_DATA_FOUND It is raised when a select into statement returns no rows. NOT_LOGGED_ON It is raised when a database call is issued without being connected to the database. PROGRAM_ERROR It is raised when PL/SQL has an internal problem. ROWTYPE_MISMATCH It is raised when a cursor fetches value in a variable having incompatible data type. STORAGE_ERROR It is raised when PL/SQL ran out of memory or memory was corrupted. TOO_MANY_ROWS It is raised when a SELECT INTO statement returns more than one row. VALUE_ERROR It is raised when an arithmetic, conversion, truncation, or sizeconstraint error occurs. ZERO_DIVIDE - It is raised when an attempt is made to divide a number by zero.

Answer 16

Eg 1: To handle an exception ‘Table not found’: delimiter $$ create procedure test() begin declare exit handler for 1146 select ‘please create the table first’ as message; select *from abc; end $$

Answer 17

There are different types of anomalies which can occur in referencing and referenced relation which can be discussed as: a). Insertion anomaly: :If a tuple is inserted in referencing relation and referencing attribute value is not present in referenced attribute b). Deletion and Updation anomaly: If a tuple is deleted or updated from referenced relation and referenced attribute value is used by referencing attribute in referencing relation, it will not allow deleting the tuple from referenced relation.

Answer 18

A functional dependency A->B in a relation holds if two tuples having same value of attribute A also have same value for attribute B.

Answer 19

Redundancy in relation may cause insertion, deletion, and update anomalies. Normalization is the process of minimizing redundancy from a relation or set of relations. Normal forms are used to eliminate or reduce redundancy in database tables.

Answer 20

Armstrong’s Axiom is a mathematical notation used to find the functional dependencies in a database. Conceived by William W. Armstrong It is a list of axioms or inference rules that can be implemented on any relational database. It is denoted by the symbol F+ primary rules axioms Axiom of Reflexivity: If A is a set of attributes and B is a subset of A, then A holds B. If B⊆A then A→B. This property is trivial property. Axiom of Augmentation: If A→B holds and Y is the attribute set, then AY→BY also holds. That is adding attributes to dependencies, does not change the basic dependencies. If A→B, then AC→BC for any C. Axiom of Transitivity: Same as the transitive rule in algebra, if A→B holds and B→C holds, then A→C also holds. A→B is called A functionally which determines B. If X→Y and Y→Z, then X→Z. SEcondary Union: If A→B holds and A→C holds, then A→BC holds. If X→Y and X→Z then X→YZ. Composition: If A→B and X→Y hold, then AX→BY holds. Decomposition: If A→BC holds then A→B and A→C hold. If X→YZ then X→Y and X→Z. Pseudo Transitivity: If A→B holds and BC→D holds, then AC→D holds. If X→Y and YZ→W then XZ→W. Self Determination: It is similar to the Axiom of Reflexivity, i.e. A→A for any A. Extensivity: Extensivity is a case of augmentation. If AC→A, and A→B, then AC→B. Similarly, AC→ABC and ABC→BC. This leads to AC→BC.

Answer 21

The Closure Of Functional Dependency means the complete set of all possible attributes that can be functionally derived from given functional dependency ● If “F” is a functional dependency then closure of functional dependency can be denoted using “{F}+ ”. ● There are three steps to calculate closure of functional dependency Step-1 : Add the attributes which are present on Left Hand Side in the original functional dependency. Step-2 : Now, add the attributes present on the Right Hand Side of the functional dependency. Step-3 : With the help of attributes present on Right Hand Side, check the other attributes that can be derived from the other given functional dependencies. Repeat this process until all the possible attributes which can be derived are added in the closure

Answer 22

A relation is in first normal form if every attribute in that relation is singled valued attribute ID Name Courses ------------------ 1 A c1, c2 2 E c3 3 M C2, c3 In the above table Course is a multi-valued attribute so it is not in 1NF. Below Table is in 1NF as there is no multi-valued attribute ID Name Course ------------------ 1 A c1 1 A c2 2 E c3 3 M c2 3 M c3

Answer 23

To be in second normal form, a relation must be in first normal form and relation must not contain any partial dependency. A relation is in 2NF if it has No Partial Dependency, i.e., no non-prime attribute (attributes which are not part of any candidate key) is dependent on any proper subset of any candidate key of the table. Prime attribute : Attributes which are a part of candidate key. * Non prime attribute :Attributes which are not a part of candidate key. * Partial Dependency : If the proper subset of candidate key determines non-prime attribute, it is called partial dependency.

Answer 24

Definition 1: A relational schema R is said to be in 3NF, First, it should be in 2NF and, no nonprime attribute should be transitively dependent on the Key of the table If X → Y and Y → Z exist then X → Z also exists which is a transitive dependency, and it should not hold. Definition 2: First it should be in 2NF and if there exists a non-trivial dependency between two sets of attributes X and Y such that X → Y (i.e., Y is not a subset of X) then either X is Super Key OR Y is a prime attribute.

Answer 25

* BCNF is the advanced version of 3NF. * A table is in BCNF if every functional dependency X->Y, X is the super key of the table. * ie, for BCNF, the table should be in 3NF, and for every FD, LHS is super key.

Answer 26

Lossless-join decomposition is a process in which a relation is decomposed into two or more relations. This property guarantees that the extra or less tuple generation problem does not occur and no information is lost from the original relation during the decomposition. It is also known as non-additive join decomposition. * When the sub relations combine again then the new relation must be the same as the original relation was before decomposition.

Answer 27

* If a relation R is decomposed into relation R1 and R2, then the dependencies of R, either must be a part of R1 or R2 or must be derivable from the combination of functional dependencies of R1 and R2. * Ie, (F1 F2 … F ∪ ∪ ∪ n)+ = F+ * Eg: Suppose there is a relation R (A, B, C, D) with functional dependency set (A->BC). The relational R is decomposed into R1(ABC) and R2(AD) which is dependency preserving because FD A->BC is a part of relation R1(ABC).

Answer 28

 Transactions are a set of operations used to perform a logical set of work.  A transaction is an action or series of actions.  It is performed by a single user to perform operations for accessing the contents of the database

Answer 29

Transactions access data using read and write operations. Read(X): Read operation is used to read the value of X from the database and stores it in a buffer in main memory. Write(X): Write operation is used to write the value back to the database from the buffer it may be possible that because of the failure of hardware, software or power, etc. that transaction may fail before finished all the operations in the set. To solve this problem, we have two important operations: 1.Commit: It is used to save the work done permanently.ie. After all instructions of a transaction are successfully executed,the changes made by transaction are made permanent in the database. 2.Rollback: It is used to undo the work done.ie, if a transaction is not able to execute all operations successfully, all changes made by that transaction is undone.

Answer 30

In order to maintain consistency in a database, before and after the transaction, certain properties are followed. These are called ACID properties. 1). Atomicity :By this, we mean that either the entire transaction takes place at once or doesn’t happen at all. There Is no midway i.e. transactions do not occur partially. Each transaction is considered as one unit and either runs to completion or is not executed at all It involves the following two operations (a) Abort : If a transaction aborts, changes made to the database are not visible. (b) Commit : If a transaction commits, changes made are visible. 2). Consistency: This means that integrity constraints must be maintained so that the database is consistent before and after the transaction. It refers to the correctness of a database. 3). Isolation: This property ensures that multiple transactions can occur concurrently without leading to the inconsistency of the database state. Transactions occur independently without interference. Changes occurring in a particular transaction will not be visible to any other transaction until that particular change in that transaction is written to memory or has been committed. 4). Durability: This property ensures that once the transaction has completed execution, the updates and modifications to the database are stored in and written to disk and they persist even if a system failure occurs. These updates now become permanent and are stored in non-volatile memory. The effects of the transaction, thus, are never lost. Property Responsibility for maintaining properties Atomicity - Transaction Manager Consistency - Application programmer Isolation - Concurrency Control Manager Durability - Recovery Manager The ACID properties provide a mechanism to ensure the correctness and consistency of a database in a way such that each transaction is a group of operations that acts as a single unit, produces consistent results, acts in isolation from other operations, and updates that it makes are durably stored. *******

Answer 31

States through which transaction goes during its lifetime. These are the states which tell about the current state of the Transaction and also tell how we will further do the processing in the transactions These are different types of Transaction States : 1. Active State– When the instructions of the transaction are running then the transaction is in active state. If all the ‘read and write’ operations are performed without any error then it goes to the “partially committed state”; if any instruction fails, it goes to the “failed state”. 2. Partially Committed– After completion of all the read and write operation the changes are made in main memory or local buffer. If the the changes are made permanent on the Data Base then the state will change to “committed state” and in case of failure it will go to the “failed state”. 3. Failed State – When any instruction of the transaction fails, it goes to the “failed state” or if failure occurs in making a permanent change of data on Data Base. 4. Aborted State – After having any type of failure the transaction goes from “failed state” to “aborted state” and since in previous states, the changes are only made to local buffer or main memory and hence these changes are deleted or rolled-back. 5. Committed State – It is the state when the changes are made permanent on the Data Base and the transaction is complete and therefore terminated in the “terminated state”. 6. Terminated State – If there isn’t any roll-back or the transaction comes from the “committed state”, then the system is consistent and ready for new transaction and the old transaction is terminated.

Answer 32

1). Recoverable Schedules: Schedules in which transactions commit only after all transactions whose changes they read commit are called recoverable schedules. In other words, if some transaction Tj is reading value updated or written by some other transaction Ti, then the commit of Tj must occur after the commit of Ti. a). Cascading Schedule A cascading schedule is classified as a recoverable schedule. A cascading rollback is a type of rollback in which if one transaction fails, then it will cause rollback of other dependent transactions. The main disadvantage of cascading rollback is that it can cause CPU time wastage. b). Cascadeless schedule When a transaction is not allowed to read data until the last transaction which has written it is committed or aborted, these types of schedules are called cascadeless schedules c). Strict schedule 2). Non Recoverable Schedule If a transaction does a dirty read operation from an uncommitted transaction and commits before the transaction from where it has read the value, then such a schedule is called an irrecoverable schedule

Answer 33

* A NoSQL (originally referring to "non-SQL" or "non-relational") provides a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relational databases. * NoSQL databases are increasingly used in big data and real time web applications. * Sometimes the data structures used by NoSQL databases are also viewed as "more flexible" than relational database tables. The data structures used by NoSQL databases are different from those used by default in relational databases, making some operations faster in NoSQL.

Answer 34

Flexible schemas * Stores unstructured, semi structured or structured data * Horizontal scaling * Handles large volume of data at high speed. * Fast queries due to the data model * Developer friendly as it is easy to update schema and fields * Applicable in real time streaming

Answer 35

graph docs key value wide column . D ocument database * A document database stores data in JSON, BSON , or XML documents (not Word documents or Google docs). In a document database, documents can be nested. Particular elements can be indexed for faster querying. * Documents can be stored and retrieved in a form that is much closer to the data objects used in applications, which means less translation is required to use the data in an application. Graph database * A graph database focuses on the relationship between data elements. Each element is stored as a node (such as a person in a social media graph). The connections between elements are called links or relationships. * A graph database is optimized to capture and search the connections between data elements, overcoming the overhead associated with JOINing multiple tables in SQL Key-Value Stores * The simplest type of NoSQL database is a key-value store . * Every data element in the database is stored as a key value pair consisting of an attribute name (or "key") and a value. Wide column store * While a relational database stores data in rows and reads data row by row, a column store is organized as a set of columns. * This means that when you want to run analytics on a small number of columns, you can read those columns directly without consuming memory with the unwanted data.

aws Flashcards

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