Database System-II

1. What is the difference between a "where" clause and a "having" clause?

Ans : "Where" is a kind of restiriction statement. You use where clause to restrict all the data from DB.Where clause is using before result retrieving. But Having clause is using after retrieving the data.Having clause is a kind of filtering command.

2. What is the basic form of a SQL statement to read data out of a table?

Ans : The basic form to read data out of table is ‘SELECT * FROM table_name; ‘ An answer: ‘SELECT * FROM table_name WHERE xyz= ‘whatever’;’ cannot be called basic form because of WHERE clause.

3. What structure can you implement for the database to speed up table reads?

Ans :  Follow the rules of DB tuning we have to: 1] properly use indexes ( different types of indexes) 2] properly locate different DB objects across different tablespaces, files and so on.3] create a special space (tablespace) to locate some of the data with special datatype ( for example CLOB, LOB and …)

4. What are the tradeoffs with having indexes?

Ans : 1. Faster selects, slower updates. 2. Extra storage space to store indexes. Updates are slower because in addition to updating the table you have to update the index.

5. What is a "join"?

Ans : ‘join’ used to connect two or more tables logically with or without common field.

6. What is "normalization"?

Ans : "Denormalization"? Why do you sometimes want to denormalize? - Normalizing data means eliminating redundant information from a table and organizing the data so that future changes to the table are easier. Denormalization means allowing redundancy in a table. The main benefit of denormalization is improved performance with simplified data retrieval and manipulation. This is done by reduction in the number of joins needed for data processing.

7. What is a "constraint"?

Ans : A constraint allows you to apply simple referential integrity checks to a table. There are four primary types of constraints that are currently supported by SQL Server: PRIMARY/UNIQUE - enforces uniqueness of a particular table column. DEFAULT - specifies a default value for a column in case an insert operation does not provide one. FOREIGN KEY - validates that every value in a column exists in a column of another table. CHECK - checks that every value stored in a column is in some specified list. Each type of constraint performs a specific type of action. Default is not a constraint. NOT NULL is one more constraint which does not allow values in the specific column to be null. And also it the only constraint which is not a table level constraint.

8. What types of index data structures can you have?

Ans :  An index helps to faster search values in tables. The three most commonly used index-types are: - B-Tree: builds a tree of possible values with a list of row IDs that have the leaf value. Needs a lot of space and is the default index type for most databases. - Bitmap: string of bits for each possible value of the column. Each bit string has one bit for each row. Needs only few space and is very fast.(however, domain of value cannot be large, e.g. SEX(m,f); degree(BS,MS,PHD) - Hash: A hashing algorithm is used to assign a set of characters to represent a text string such as a composite of keys or partial keys, and compresses the underlying data. Takes longer to build and is supported by relatively few databases.

Java Fundmental-VII

1. Do I need to use synchronized on setValue(int)?

Ans : It depends whether the method affects method local variables, class static or instance variables. If only method local variables are changed, the value is said to be confined by the method and is not prone to threading issues.
2. Do I need to use synchronized on setValue(int)?

Ans : It depends whether the method affects method local variables, class static or instance variables. If only method local variables are changed, the value is said to be confined by the method and is not prone to threading issues.
3. What is the SwingUtilities.invokeLater(Runnable) method for?

Ans : The static utility method invokeLater(Runnable) is intended to execute a new runnable thread from a Swing application without disturbing the normal sequence of event dispatching from the Graphical User Interface (GUI). The method places the runnable object in the queue of Abstract Windowing Toolkit (AWT) events that are due to be processed and returns immediately. The runnable object’s run() method is only called when it reaches the front of the queue. The deferred effect of the invokeLater(Runnable) method ensures that any necessary updates to the user interface can occur immediately, and the runnable work will begin as soon as those high priority events are dealt with. The invoke later method might be used to start work in response to a button click that also requires a significant change to the user interface, perhaps to restrict other activities, while the runnable thread executes.
4. What is the volatile modifier for?

Ans : The volatile modifier is used to identify variables whose values should not be optimized by the Java Virtual Machine, by caching the value for example. The volatile modifier is typically used for variables that may be accessed or modified by numerous independent threads and signifies that the value may change without synchronization.
5. Which class is the wait() method defined in?

Ans : The wait() method is defined in the Object class, which is the ultimate superclass of all others. So the Thread class and any Runnable implementation inherit this method from Object. The wait() method is normally called on an object in a multi-threaded program to allow other threads to run. The method should should only be called by a thread that has ownership of the object’s monitor, which usually means it is in a synchronized method or statement block.
6. Which class is the wait() method defined in?

Ans : I get incompatible return type for my thread’s getState( ) method! - It sounds like your application was built for a Java software development kit before Java 1.5. The Java API Thread class method getState() was introduced in version 1.5. Your thread method has the same name but different return type. The compiler assumes your application code is attempting to override the API method with a different return type, which is not allowed, hence the compilation error.
7. What is a working thread?

Ans : A working thread, more commonly known as a worker thread is the key part of a design pattern that allocates one thread to execute one task. When the task is complete, the thread may return to a thread pool for later use. In this scheme a thread may execute arbitrary tasks, which are passed in the form of a Runnable method argument, typically execute(Runnable). The runnable tasks are usually stored in a queue until a thread host is available to run them. The worker thread design pattern is usually used to handle many concurrent tasks where it is not important which finishes first and no single task needs to be coordinated with another. The task queue controls how many threads run concurrently to improve the overall performance of the system. However, a worker thread framework requires relatively complex programming to set up, so should not be used where simpler threading techniques can achieve similar results.
8. What is a green thread?

Ans : A green thread refers to a mode of operation for the Java Virtual Machine (JVM) in which all code is executed in a single operating system thread. If the Java program has any concurrent threads, the JVM manages multi-threading internally rather than using other operating system threads. There is a significant processing overhead for the JVM to keep track of thread states and swap between them, so green thread mode has been deprecated and removed from more recent Java implementations. Current JVM implementations make more efficient use of native operating system threads.
9. What are native operating system threads?

Ans : Native operating system threads are those provided by the computer operating system that plays host to a Java application, be it Windows, Mac or GNU/Linux. Operating system threads enable computers to run many programs simultaneously on the same central processing unit (CPU) without clashing over the use of system resources or spending lots of time running one program at the expense of another. Operating system thread management is usually optimised to specific microprocessor architecture and features so that it operates much faster than Java green thread processing.