Q. Consider the following employee database- Employee (Emp_name, Street, City) Works (Emp_name, Company_name, Salary) Company (Company_name, City) Manages (Emp_name, manager_name) Write expressions in SQL for the following queries- (i) Find all employees who live in the city where the company for which they work is located. (ii) Find all employees who live in the same […]

Concurrency control is the procedure in DBMS for managing simultaneous operations without conflicting with each another. Concurrent access is quite easy if all users are just reading data. There is no way they can interfere with one another. Though for any practical database, would have a mix of reading and WRITE operations and hence the […]

What is Cursor ? Cursor is a pointer in memory area called context area. What is context area ? Context area is a memory area inside the Process Global Area (PGA) which helps Oracle server in processing an SQL statement by holding the important information about the statement. This information include: Rows returned by a […]

What is Trigger ? A trigger is defined as an action taken by database when some database related events occur. Triggers are implicitly fired by ORACLE when triggering event occur,no matter which user is using or used by which application. The code within the trigger called the trigger body. Trigger fires in following operations DML […]

Five Important aggregate functions are SUM, AVG, MIN, MAX and COUNT. They are called aggregate functions because they summarize the results of a query, rather than listing all of the rows. SUM () gives the total of all the rows, satisfying any conditions, of the given column, where the given column is numeric. AVG () […]

A SQL Join statement is used to combine data or rows from two or more tables based on a common field between them. Different types of Joins are: 1. INNER JOIN Select records that have matching values in both tables. 2. LEFT JOIN Select records from the first (left-most) table with matching right table records. […]

A DBMS based on relational model is called relational database management system or RDBMS.RDBMS uses relational structures to store data. A relation is also called a table.A relation contain tuples (rows) and attributes (columns). STUDENT Roll_no Name City 10001 Anurag Burhanpur 10002 Yash Khandwa 10003 Praful Nepa nagar 10004 Harshal Khanknar MARKS Roll_no Marks 10001 […]

OODBMS RDBMS Stores data and methods Stores only data Main objective data encapsulation and data independence Main objective data independence Classes can be reorganized without affecting the mode of using them. Data can be reorganized without affecting the mode of using them. here objects are active Here data are passive Here structure of data is […]

What is Serializability ? Serializability is the concurrency scheme.  Serializability ensures that a schedule for executing concurrent transactions is equivalent to one that executes the transactions serially in some order.  It assumes that all accesses to the database are done using read and write operations. Let’s take a simple example: In this SQL query, two […]

What are Schedules ? Schedules are sequences that indicate the chronological order in which instructions of concurrent transactions are executed.A schedule must preserve the order in which the instructions appear in each individual transaction. Types of schedule Serial schedule Non serial schedule Example of serial schedule Transaction2 starts after completion of transaction1. Transaction1 Transaction2 Read(X); […]