SQL queries are combined using the UNION operator. Using UNION, multiple SELECT statements can be specified, and their results can be combined into a single result set.
Using UNION
Using UNION is simple enough. All you do is specify each SELECT statement and place the keyword UNION between each.
Let's look at an example. You need a report on all your customers in Illinois, Indiana, and Michigan. You also want to include all Fun4All locations, regardless of state. Of course, you can create a WHERE clause that will do this, but this time you'll use a UNION instead.
As I just explained, creating a UNION involves writing multiple SELECT statements. First look at the individual statements:
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_state IN ('IL','IN','MI');
cust_name cust_contact cust_email
----------- ------------- ------------
Village Toys John Smith sales@villagetoys.com
Fun4All Jim Jones jjones@fun4all.com
The Toy Store Kim Howard NULL
Now Combne
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_state IN ('IL','IN','MI')
UNION
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_name = 'Fun4All';
cust_name cust_contact cust_email
--------- ------------ ----------
Fun4All Denise L. Stephens dstephens@fun4all.com
Fun4All Jim Jones jjones@fun4all.com
Village Toys John Smith sales@villagetoys.com
The Toy Store Kim Howard NULL
Monday, May 2, 2011
what is Cursors in Oracle , Cursors and Web-Based Applications
SQL retrieval operations work with sets of rows known as result sets. The rows returned are all the rows that match a SQL statement—zero or more of them. Using simple SELECT statements, there is no way to get the first row, the next row, or the previous 10 rows. This is an integral part of how a relational DBMS worksDifferent DBMSs support different cursor options and features. Some of the more common ones are:
The capability to flag a cursor as read-only so that data can be read but not updated or deleted
The capability to control the directional operations that can be performed (forward, backward, first, last, absolute position, relative position, and so on)
The capability to flag some columns as editable and others as not editable
Scope specification so as to be able to make the cursor accessible to a specific request that created it (a stored procedure, for example) or to all requests
Instructing the DBMS to make a copy of the retrieved data (as opposed to pointing to the live data in the table) so that data does not change between the time the cursor is opened and the time it is accessed
Cursors and Web-Based Applications Cursors are rather useless when it comes to Web-based applications (ASP, ColdFusion, PHP, and JSP, for example). Cursors are designed to persist for the duration of a session between a client application and a server, but this client/server model does not fit in the Web application world because the application server is the database client, not the end user. As such, most Web application developers avoid the use of cursors and re-create the functionality themselves if needed.
The capability to flag a cursor as read-only so that data can be read but not updated or deleted
The capability to control the directional operations that can be performed (forward, backward, first, last, absolute position, relative position, and so on)
The capability to flag some columns as editable and others as not editable
Scope specification so as to be able to make the cursor accessible to a specific request that created it (a stored procedure, for example) or to all requests
Instructing the DBMS to make a copy of the retrieved data (as opposed to pointing to the live data in the table) so that data does not change between the time the cursor is opened and the time it is accessed
Cursors and Web-Based Applications Cursors are rather useless when it comes to Web-based applications (ASP, ColdFusion, PHP, and JSP, for example). Cursors are designed to persist for the duration of a session between a client application and a server, but this client/server model does not fit in the Web application world because the application server is the database client, not the end user. As such, most Web application developers avoid the use of cursors and re-create the functionality themselves if needed.
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What is RDBMS?
In recent years, database management systems (DBMS) have established themselves as the primary means of data storage for information systems ranging from large commercial transaction processing applications to PC-based desktop applications. At the heart of most of today's information systems is a relational database management system (RDBMS). RDBMSs have been the workhorse for data management operations for over a decade and continue to evolve and mature, providing sophisticated storage, retrieval, and distribution functions to enterprise-wide data processing and information management systems. Compared to the file systems, relational database management systems provide organizations with the capability to easily integrate and leverage the massive amounts of operational data into meaningful information systems. The evolution of high-powered database engines such as Oracle7 has fostered the development of advanced "enabling" technologies including client/server, data warehousing, and online analytical processing, all of which comprise the core of today's state-of-the-art information management systems.
Examine the components of the term relational database management system. First, a database is an integrated collection of related data. Given a specific data item, the structure of a database facilitates the access to data related to it, such as a student and all of his registered courses or an employee and his dependents. Next, a relational database is a type of database based in the relational model; non-relational databases commonly use a hierarchical, network, or object-oriented model as their basis. Finally, a relational database management system is the software that manages a relational database. These systems come in several varieties, ranging from single-user desktop systems to full-featured, global, enterprise-wide systems, such as Oracle7.
This blog discusses the basic elements of a relational database management system, the relational database, and the software systems that manage it. Also included is a discussion of nonprocedural data access. If you are a new user to relational database technology, you'll have to change your thinking somewhat when it comes to referencing data nonprocedurally.
The Relational Database Model
Most of the database management systems used by commercial applications today are based on one of three basic models: the hierarchical model, the network model, or the relational model. The following sections describe the various differences and similarities of the models.
Hierarchical and Network Models
The first commercially available database management systems were of the CODASYL type, and many of them are still in use with mainframe-based, COBOL applications. Both network and hierarchical databases are quite complex in that they rely on the use of permanent internal pointers to relate records to each other. For example, in an accounts payable application, a vendor record might contain a physical pointer in its record structure that points to purchase order records. Each purchase order record in turn contains pointers to purchase order line item records.
The process of inserting, updating, and deleting records using these types of databases requires synchronization of the pointers, a task that must be performed by the application. As you might imagine, this pointer maintenance requires a significant amount of application code (usually written in COBOL) that at times can be quite cumbersome.
Elements of the Relational Model
Relational databases rely on the actual attribute values as opposed to internal pointers to link records. Instead of using an internal pointer from the vendor record to purchase order records, you would link the purchase order record to the vendor record using a common attribute from each record, such as the vendor identification number.
Although the concepts of academic theory underlying the relational model are somewhat complex, you should be familiar with are some basic concepts and terminology. Essentially, there are three basic components of the relational model: relational data structures, constraints that govern the organization of the data structures, and operations that are performed on the data structures.
Relational Data Structures
The relational model supports a single, "logical" structure called a relation, a two-dimensional data structure commonly called a table in the "physical" database. Attributes represent the atomic data elements that are related by the relation. For example, the Customer relation might contain such attributes about a customer as the customer number, customer name, region, credit status, and so on.
Key Values and Referential Integrity
Attributes are grouped with other attributes based on their dependency on a primary key value. A primary key is an attribute or group of attributes that uniquely identifies a row in a table. A table has only one primary key, and as a rule, every table has one. Because primary key values are used as identifiers, they cannot be null. Using the conventional notation for relations, an attribute is underlined to indicate that it is the primary key of the relation. If a primary key consists of several attributes, each attribute is underlined.
You can have additional attributes in a relation with values that you define as unique to the relation. Unlike primary keys, unique keys can contain null values. In practice, unique keys are used to prevent duplication in the table rather than identify rows. Consider a relation that contains the attribute, United States Social Security Number (SSN). In some rows, this attribute may be null in since not every person has a SSN; however for a row that contains a non-null value for the SSN attribute, the value must be unique to the relation.
Linking one relation to another typically involves an attribute that is common to both relations. The common attributes are usually a primary key from one table and a foreign key from the other. Referential integrity rules dictate that foreign key values in one relation reference the primary key values in another relation. Foreign keys might also reference the primary key of the same relation. Figure illustrates two foreign key relationships.
Oracle and Client/Server
Oracle Corporation's reputation as a database company is firmly established in its full-featured, high-performance RDBMS server. With the database as the cornerstone of its product line, Oracle has evolved into more than just a database company, complementing its RDBMS server with a rich offering of well-integrated products that are designed specifically for distributed processing and client/server applications. As Oracle's database server has evolved to support large-scale enterprise systems for transaction processing and decision support, so too have its other products, to the extent that Oracle can provide a complete solution for client/server application development and deployment. This chapter presents an overview of client/server database systems and the Oracle product architectures that support their implementation.
An Overview of Client/Server Computing
The premise of client/server computing is to distribute the execution of a task among multiple processors in a network. Each processor is dedicated to a specific, focused set of subtasks that it performs best, and the end result is increased overall efficiency and effectiveness of the system as a whole. Splitting the execution of tasks between processors is done through a protocol of service requests; one processor, the client, requests a service from another processor, the server. The most prevalent implementation of client/server processing involves separating the user interface portion of an application from the data access portion.
On the client, or front end, of the typical client/server configuration is a user workstation operating with a Graphical User Interface (GUI) platform, usually Microsoft Windows, Macintosh, or Motif. At the back end of the configuration is a database server, often managed by a UNIX, Netware, Windows NT, or VMS operating system.
Client/server architecture also takes the form of a server-to-server configuration. In this arrangement, one server plays the role of a client, requesting database services from another server. Multiple database servers can look like a single logical database, providing transparent access to data that is spread around the network.
Designing an efficient client/server application is somewhat of a balancing act, the goal of which is to evenly distribute execution of tasks among processors while making optimal use of available resources. Given the increased complexity and processing power required to manage a graphical user interface (GUI) and the increased demands for throughput on database servers and networks, achieving the proper distribution of tasks is challenging. Client/server systems are inherently more difficult to develop and manage than traditional host-based application systems because of the following challenges:
The components of a client/server system are distributed across more varied types of processors. There are many more software components that manage client, network, and server functions, as well as an array of infrastructure layers, all of which must be in place and configured to be compatible with each other.
The complexity of GUI applications far outweighs that of their character-based predecessors. GUIs are capable of presenting much more information to the user and providing many additional navigation paths to elements of the interface.
Troubleshooting performance problems and errors is more difficult because of the increased number of components and layers in the system.
Databases in a Client/Server Architecture
Client/server technologies have changed the look and architecture of application systems in two ways. Not only has the supporting hardware architecture undergone substantial changes, but there have also been significant changes in the approach to designing the application logic of the system.
Prior to the advent of client/server technology, most Oracle applications ran on a single node. Typically, a character-based SQL*Forms application would access a database instance on the same machine with the application and the RDBMS competing for the same CPU and memory resources. Not only was the system responsible for supporting all the database processing, but it was also responsible for executing the application logic. In addition, the system was burdened with all the I/O processing for each terminal on the system; each keystroke and display attribute was controlled by the same processor that processed database requests and application logic.
Client/server systems change this architecture considerably by splitting all of the interface management and much of the application processing from the host system processor and distributing it to the client processor.
Combined with the advances in hardware infrastructure, the increased capabilities of RDBMS servers have also contributed to changes in the application architecture. Prior to the release of Oracle7, Oracle's RDBMS was less sophisticated in its capability to support the processing logic necessary to maintain the integrity of data in the database. For example, primary and foreign key checking and enforcement was performed by the application. As a result, the database was highly reliant on application code for enforcement of business rules and integrity, making application code bulkier and more complex. Figure 2.1 illustrates the differences between traditional host-based applications and client/server applications. Client/server database applications can take advantage of the Oracle7 server features for implementation of some of the application logic.
