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a level coursework Clifton College

For the sake of our discussion a relational database is a persistent storage mechanism that enables you to both store data and optionally implement functionality. The goal of this article is to overview relational database management system (RDBMS) technology and to explore the practical issues applicable to its use in a level coursework Clifton College, modern organizations; the goal is not to discuss relational theory. RDBMSs are used to reading critical thinking city, university of london, store the a level coursework Clifton College, information required by essays to write about university of central florida applications built using procedural technologies such as COBOL or FORTRAN, object technologies such as Java and C#, and how to write a thesis statement for a narrative essay rossall school, component-based technologies such as Visual Basic. a level coursework Clifton College! Because RDBMSs are the dominant persistent storage technology it is critical that all IT professionals understand at least the a level coursework Clifton College, basics of RDBMSs, the a level coursework Clifton College, challenges surrounding the essays to write about university of central florida, technology, and when it is appropriate to use them. Tables typically have keys, one or more columns that uniquely identify a row within the table, in the case of the Customer table the key would be CustomerNumber . To improve access time to a data table you define an reading critical thinking city, university of london index on the table. How to write a thesis statement for a narrative essay Rossall School! An index provides a quick way to look up data based on one or more columns in the table, just like the index of a level coursework Clifton College a book enables you to find specific information quickly. The easiest way to manipulate a relational database is to submit Structured Query Language (SQL) statements to it. Figure 1 depicts a simple data model, see Data Modeling 101 or better yet Agile Data Modeling, using the a level coursework Clifton College, proposed UML data modeling notation. To create a row in the a level coursework Clifton College, Seminar table you would issue an short journal articles abingdon school INSERT statement, an example of which is shown in how to write a thesis statement for a narrative essay rossall school, Code Figure 1. Similarly, Code Figure 2 presents and example of a level coursework Clifton College how to read a row by a level coursework Clifton College issuing a SELECT statement. Essays to write about University of Central Florida! Code Figure 3 shows how to update an existing row via an UPDATE statement and Code Figure 4 how to delete a row with a DELETE statement. All four of these examples were taken, as well as the data model, were adapted from The Object Primer. Reading critical thinking City, University of London! A very good resource for a level coursework Clifton College, learning SQL is SQL Queries for Mere Mortals by Micheal J. Hernandez and John L. Viescas. Code Figure 1. SQL statement to insert a row into the "Seminar" table. (SEMINAR_ID, COURSE_ID, OVERSEER_ID, SEMINAR_NUMBER) VALUES (74656, 1234, ‘THX0001138’, 2) Code Figure 2. SQL statement to retrieve a row from the "Seminar" table. WHERE SEMINAR_ID = 1701. Code Figure 3. SQL statement to update a row in a table in the "Seminar" table. SET OVERSEER_ID = ‘NCC0001701’, SEMINAR_NUMBER = 3. WHERE SEMINAR_ID = 1701. Code Figure 4. Essays to write about University of Central Florida! SQL statement to delete data from the "Seminar" table. WHERE SEMINAR_ID > 1701. AND OVERSEER_ID = ‘THX0001138’ Object storage . To store an object in a relational database you need to flatten it – create a data representation of the object – because relational databases only store data. a level coursework Clifton College! To retrieve the object you would read the data from the database and then create the object, often referred to as restoring the object, based on that data. Although storing objects in a relational database sounds like a simple thing to reading critical thinking city, university of london, achieve, practice shows that it isn’t. This is due to the object-relational impedance mismatch, the fact that relational database technology and object technology are based on different underlying theories, a topic discussed in The Object-Relational (O/R) Impedance Mismatch. To store objects successfully in relational databases you need to learn how to map your object schema to your relational database schema. Implementing behavior within the database . Behavior is implemented in a level coursework Clifton College, a relational database via stored procedures and/or stored functions that can be invoked internally within the database and a level coursework Clifton College, often by external applications. Stored functions and procedures are operations that run within an a level coursework Clifton College RDBMS, the difference being what the operation can return and whether it can be invoked in a query. The differences aren’t important for our purposes so the term “stored procedure” will be used to refer to both stored functions and stored procedures. In the past stored procedures were written in where can i pay someone to write my essay anglia ruskin university (navitas), a proprietary language, such as Oracle’s PL/SQL, although now Java is quickly becoming the language of choice for database programming. A stored procedure typically runs some SQL code, massages the data, and then hands back a response in the form of zero or more records, or a response code, or as a database error message. Effective use of stored procedures is discussed in a level coursework Clifton College, detail in Implementation Strategies for Persisting Objects in RDBs. Concurrency control . Consider an a level coursework Clifton College airline reservation system. There is a flight with one seat left on it, and two people are trying to reserve that seat at the same time. Both people check the flight status and are told that a seat is still available. Both enter their payment information and a level coursework Clifton College, click the reservation button at the same time. What should happen? If the system is working properly only one person should be given a seat and the other should be told that there is no longer one available. Concurrency control is what makes this happen. Concurrency control must be implemented throughout your object source code and within your database. Transaction control . A transaction is where can i pay someone to write my essay anglia ruskin university (navitas) a collection of actions on your database – such as the saving of, retrieval of, or deletion of data – which form a work unit. A flat transactions is an “all-or-nothing” approach where all the actions must either succeed or be rolled back (canceled). A nested transaction is an a level coursework Clifton College approach where some of the actions are transactions in their own right. Where can i pay someone to write my essay Anglia Ruskin University (Navitas)! These sub-transactions are committed once successful and are not rolled back if the larger transaction fails. Transactions may also be short-lived, running in thousandths of a second, or long-lived, taking hours, days, weeks, or even months to complete. Transaction control is a critical concept for all developers to understand. Enforcing referential integrity . Referential integrity (RI) is the assurance that a reference from where can i pay someone to write my essay anglia ruskin university (navitas), one entity to a level coursework Clifton College, another entity is introduction to argumentative essay international college of management (icms) valid. For example, if a customer references an address, then that address must exist. If the address is deleted then all references to it must also be removed, otherwise your system must not allow the deletion. Contrary to a level coursework Clifton College, popular belief, RI isn’t just a database issue, it’s an issue for your entire system. A customer is implemented as an object within a Java application and as one or more records in your database – addresses are also implemented as objects and as rows. To delete an address, you must remove the where can i pay someone to write my essay anglia ruskin university (navitas), address object from memory, any direct or indirect references to essays to write about university of central florida, it (an indirect reference to an a level coursework Clifton College address would include a customer object knowing the where can i pay someone to write my essay anglia ruskin university (navitas), value of the AddressID , the primary key of the address in the database), the address row(s) from your database, and any references to it (via foreign keys) in your database. To complicate matters, if you have a farm of application servers that address object could exist simultaneously on notes on essay writing seneca college several machines. Furthermore, if you have other applications accessing your database then it is possible that they too have representations of the address in their memory as well. Worse yet, if the address is a level coursework Clifton College stored in several places (e.g. different databases) you should also consider taking this into reading critical thinking city, university of london account. All developers should understand basic strategies for implementing referential integrity. a level coursework Clifton College! Table 1 describes the common technical features found in RDBMS products, potential ways that developers will use them, and the potential drawbacks associated with their use. Database cursors – A database cursor is effectively a handle to the results of a level coursework Clifton College a SQL query, enabling you to short journal articles abingdon school, move forward and a level coursework Clifton College, backward through the result set one or more records at a time. Accessing large results sets in smaller portions enables your application to display initial results earlier, increasing response time. Performance is improved when a portion of a result set is required because less data is transmitted across the network. Application developers need to understand that the underlying data can change between the times that data records are accessed via the cursor: previously retrieved records may have been deleted, records may have been inserted into previously retrieved portions of the a level coursework Clifton College, result set, or previously retrieved records may have been modified. Not all cursors are created equal. Some cursors only allow forward scrolling. a level coursework Clifton College! Cursors are a resource drain on the where can i pay someone to write my essay anglia ruskin university (navitas), database because they are memory intensive. Java – Most database vendors support a Java VM within the database. Development of relatively platform independent behavior in the database. Development of data intensive behavior that results in notes on essay writing seneca college, a relatively small return value. Encapsulation of database access to support security access control to information. Implementation of shared behavior required by many applications. Different version of VMs between application server and database server increases complexity of development. Behavior implemented in the database can easily become a bottleneck. Triggers – A trigger is a procedure that is where can i pay someone to write my essay anglia ruskin university (navitas) run either before or after an action (such as a create, update, or delete) is performed on a row in a database table. Enforce referential integrity within your database. These types of triggers can often be automatically generated by your data modeling or database administration tool. Often a lowest common denominator for implementing referential integrity constraints. Perform hand-crafted audit logging. Hand-crafted, or hand-modified, triggers can be difficult to maintain and will increase your dependency on your database vendor. Triggers are typically implemented in a proprietary language requiring an extra skillset on your team. Because triggers are automatically invoked they can be very dangerous (such as “uncontrolled” cascading deletions resulting from chained delete triggers). Behavior implemented in the notes on essay writing seneca college, database can easily become a bottleneck if your database doesn’t scale well. Your application source code . a level coursework Clifton College! When you change your database schema you must also change the source code within your application that accesses the changed portion of the schema. Figure 2 depicts the best-case scenario – when it is only your application code that is short journal articles abingdon school coupled to your database schema. This situation is a level coursework Clifton College traditionally referred to as a stovepipe. These situations do exist and are often referred to as stand-alone applications, stovepipe systems, or greenfield projects. Count yourself lucky if this is a level coursework Clifton College your situation because it is very rare in a level coursework Clifton College, practice. Other application source code . Figure 3 depicts the worst-case scenario for relational databases – a wide variety of software systems are coupled to your database schema, a situation that is short journal articles abingdon school quite common with existing production databases. It is quite common to find that in addition to the application that your team is currently working on a level coursework Clifton College that other applications, some of which you know about and some of which you don’t, are also coupled to a level coursework Clifton College, your database. Perhaps an online system reads and writes to your database. Perhaps a manager has written a spreadsheet, unbeknownst to you, that reads data from your database that she uses to summarize information critical to her job. Data load source code . Data loads from other sources, such as government provided tax tables or your own test data, are often coupled to your database schema. Data extract source code . There may be data extraction scripts or programs that read data from your database, perhaps to produce an a level coursework Clifton College XML data file or simply so your data can be loaded into another database. Persistence frameworks/layers . A persistence framework encapsulates the logic for how to write a thesis statement for a narrative essay rossall school, mapping application classes to persistent storage sources such as your database. When you refactor your database schema you will need to update the meta data, or the source code as the case may be, which describes the mappings. Reading critical thinking City, University of London! Itself . Coupling exists within your database. Essays to write about University of Central Florida! A single column is a level coursework Clifton College coupled to any stored procedure that references it, other tables that use the a level coursework Clifton College, column as a foreign key, any view that references the where can i pay someone to write my essay anglia ruskin university (navitas), column, and so on. Notes on essay writing Seneca College! A simple change could result in several changes throughout your database. Data migration scripts . Changes to how to write a thesis statement for a narrative essay rossall school, your database schema will require changes to your data migration scripts. Test code . Testing code includes any source code that puts your database into how to write a thesis statement for a narrative essay rossall school a known state, that performs transactions that affect your database, and that reads the results from the database to compare it against expected results. Clearly this code may need to be updated to how to write a thesis statement for a narrative essay rossall school, reflect any database schema changes that you make. Documentation . Some of the most important documentation that you are likely to keep pertains to your physical database schema, including but not limited to physical data models and descriptive meta data. When your database schema changes the documentation describing will also need to change accordingly. Although Agile Modeling implores you to Update Only When It Hurts, that your documentation doesn’t have to be perfectly in synch with your schema at all times, the reality is that you will need to update your docs at some point. As you can see, coupling is a serious issue when it comes to relational databases. a level coursework Clifton College! To make matters worse the concept of coupling is virtually ignored within database theory circles. Although most database theory books will cover data normalization in excruciating detail, I argue that normalization is the data community’s way of addressing cohesion. a level coursework Clifton College! My experience is that coupling becomes a serious issue only when you start to consider behavioral issues (e.g. code), something that traditional database theory chooses not to address. This is another reason to follow Agile Modeling (AM)’s Multiple Models principle and look beyond data (Ambler 2002a). Performance issues are difficult to predict . When you are working with a shared database, such as the situation implied in Figure 3, you may find that the performance characteristics of introduction to argumentative essay international college of management (icms) your database are hard to predict because each application accesses the database in its own unique way. For example, perhaps one legacy application updates information pertaining to items for sale sporadically throughout the month, enabling a human operator to add new items or update existing ones, activity that doesn’t really affect your application’s performance in a meaningful way. However, this same application also performs batch loads of items available for sale via other companies that you have partnered with, items that you want to carry on your web site as soon as they are available. These batch loads can take several minutes, during which period the reading critical thinking city, university of london, Item table is a level coursework Clifton College under heavy load and thus your online application is potentially affected. Data integrity is difficult to introduction to argumentative essay international college of management (icms), ensure with shared databases . Because no single application has control over the data it is very difficult to be sure that all applications are operating under the same business principles. Essays to write about University of Central Florida! For example, your application may consider an order as fulfilled once it has been shipped and a payment has been received. The original legacy application that is still in use by your customer support representatives to a level coursework Clifton College, take orders over the phone may consider an order as fulfilled once it has been shipped, the payment received, and the payment deposited into your bank account. A slight difference in the way that a fundamental business rule has been implemented may have serious business implications for any application that accesses the reading critical thinking city, university of london, shared databases. a level coursework Clifton College! Less subtly, imagine what would happen if your online order taking application calculates the total for a level coursework Clifton College, an order and how to write a thesis statement for a narrative essay rossall school, stores it in the a level coursework Clifton College, order table, whereas the legacy application calculates the subtotals only for order items but does not total the a level coursework Clifton College, order. When the order fulfillment application sees an order with no total it calculates the a level coursework Clifton College, total, and appropriate taxes, whereas if a total already exists it uses the existing figure. If a customer makes an order online and introduction to argumentative essay international college of management (icms), then calls back a few hours later and a level coursework Clifton College, has one of your customer service representatives modify their existing order, perhaps to add several items to it, the order total is no longer current because it has not been updated properly. Referential integrity issues such as this are covered in detail in the a level coursework Clifton College, Implementing Referential Integrity article. Operational databases require different design strategies than reporting databases . The schemas of operational databases reflect the operational needs of the applications that access them, often resulting in a reasonably normalized schema with some portions of it denormalized for performance reasons. Reporting databases, on the other hand, are typically highly denormalized with significant data redundancy within them to support a wide range of reporting needs. Every technology has its strengths and weaknesses, and RDBMS technology is not an exception to this rule. Luckily there are ways that you can mitigate some of these challenges, and encapsulation is an important technique to do so. By encapsulating access to a database, perhaps through something as simple as data access objects or something as complex as a persistence framework, you can reduce the coupling that your database is involved with. The Implementation Strategies for a level coursework Clifton College, Persisting Objects in RDBs chapter compares and contrasts various encapsulation strategies that you have available to you. For now assume that it is notes on essay writing seneca college possible to hide the essays to write about university of central florida, details of your database schema from the majority of the developers within your organization while at the same time giving them access to your database. Some people, often just the a level coursework Clifton College, Agile DBA(s) responsible for notes on essay writing seneca college, supporting the a level coursework Clifton College, database, will need to understand and a level coursework Clifton College, work with the reading critical thinking city, university of london, underlying schema to a level coursework Clifton College, maintain and evolve the a level coursework Clifton College, encapsulation strategy. One advantage of how to write a thesis statement for a narrative essay rossall school encapsulating access to a level coursework Clifton College, your database is that it enables application programmers to focus on the short journal articles abingdon school, business problem itself. Let’s assume we’re doing something simple such as data access objects that implement the SQL code to access your database schema. The application programmers will work with these data access classes, not the database. This enables your Agile DBA to evolve the database schema as they need to, perhaps via database refactorings, and all they need to worry about is keeping the data access classes up to date. Introduction to argumentative essay International College of Management (ICMS)! This reveals a second advantage to this approach – it provides greater freedom to Agile DBAs to do their job. Figure 4 depicts the concept of encapsulating access to your database, showing how the best case scenario of Figure 2 and the worst case scenario of Figure 3 would likely change. In the best-case scenario your business source code would interact with the data access objects that in turn would interact with the database. The primary advantage would be that all of the a level coursework Clifton College, data-related code would be in one place, making it easier to modify whenever database schema changes occurred or to support performance-related changes. a level coursework Clifton College! It’s interesting to how to write a thesis statement for a narrative essay rossall school, note that the business code that your application programmers are writing would still be coupled to the data access objects. Therefore they’d need to change their code whenever the a level coursework Clifton College, interface of a data access object changed. You’ll never get away from coupling. However, from the point of view of the application programmer this is a much easier change to detect and act on where can i pay someone to write my essay anglia ruskin university (navitas) – with the a level coursework Clifton College, database encapsulation strategy in place the application programmers are only reading critical thinking city, university of london dealing with program source code (e.g. Java) and not program source code plus SQL code. Things aren’t quite so ideal for the a level coursework Clifton College, worst-case scenario. Although it is possible that all applications could take advantage of your encapsulation strategy the reality is short journal articles abingdon school that only a subset will be able to. Platform incompatibility can be an issue – perhaps your data access objects are written in short journal articles abingdon school, Java but some legacy applications are written using technologies that can’t easily access Java. Perhaps you’ve chosen not to rework some of your legacy applications simply to use the database encapsulation strategy. Perhaps some applications already have an encapsulation strategy in place (if so, you might want to consider reusing the existing strategy instead of building your own). Perhaps you want to use technologies, such as a bulk load facility, that require direct access to the database schema. The point to where can i pay someone to write my essay anglia ruskin university (navitas), be made is that a portion of your organization’s application will be able to take advantage of your encapsulation strategy and a level coursework Clifton College, some won’t. There is a level coursework Clifton College still a benefit to doing this, you are reducing coupling and therefore reducing your development costs and maintenance burden, the problem is that it isn’t a fully realized benefit. Another advantage of encapsulating access to a database is that it gives you a common place, in addition to the database itself, to implement data-oriented business rules. Object/relational databases . Object/relational databases (ORDBs), or more properly object/relational database management systems (ORDBMSs), add new object storage capabilities to RDBMSs. ORDBs, add new facilities to integrate management of traditional fielded data, complex objects such as time-series and geo-spatial data and diverse binary media such as audio, video, images, and (sometimes) Java applets. ORDBMSs basically add to RDBMSs features such as defined data types, for example you could define a data type called SurfaceAddress that has all of the attributes and behaviors of an address, as well as the ability to navigate objects in addition to an RDBMS’s ability to join tables. By implementing objects within the database, an ORDBMS can execute complex analytical and data manipulation operations to reading critical thinking city, university of london, search and transform multimedia and other complex objects. ORDBs support the robust transaction and data-management features of RDBMSs while at the a level coursework Clifton College, same time offer a limited form of the flexibility of a level coursework Clifton College object-oriented databases. Because of ORDBMSs relational foundation, database administrators work with familiar tabular structures and data definition languages (DDLs) and programmers access them via familiar approaches such as SQL3, JDBC, and proprietary call interfaces. Object databases . Object databases (ODBs), also known as object-oriented databases (OODBs) or object-oriented database management systems (OODBMSs), nearly seamlessly add database/persistence functionality to object programming languages. In other words, full-fledged objects are implemented in the database. They bring much more than persistent storage of programming language objects. Reading critical thinking City, University of London! ODBs extend the semantics of Java to a level coursework Clifton College, provide full-featured database programming capability, via new class libraries specific to the ODB vendor, while retaining native language compatibility. a level coursework Clifton College! A major benefit of this approach is the unification of the application and database development into a seamless model. As a result, applications require less code, use more natural persistence modeling, and introduction to argumentative essay international college of management (icms), code bases are easier to maintain. Object-oriented developers can write complete database applications with a modest amount of additional effort without the need to marshal their objects into reading critical thinking city, university of london flatten data structures for storage, as a result forgoing the marshalling overhead inherent with other persistence mechanism technologies (such as RDBs). a level coursework Clifton College! This one-to-one mapping of application objects to a level coursework Clifton College, database objects provides higher performance management of objects and enables better management of the complex interrelationships between objects. a level coursework Clifton College! Native XML databases . Notes on essay writing Seneca College! Native XML databases store information as XML documents following one of two approaches: First, a native XML database will either store a modified form of the short journal articles abingdon school, entire XML document in the file system, perhaps in notes on essay writing seneca college, a compressed or pre-parsed binary form. Second, a native XML database may opt to map the structure of the document to the database, for example mapping the Document Object Model (DOM) to internal structures such as Elements, Attributes, and Text – exactly what is mapped depends on the database. The most important difference between these approaches, from the point of view of an application developer, is the way they are accessed: with the first approach the only interface to the data is XML and related technologies, such as XPath (a language design specifically for short journal articles abingdon school, addressing parts of an XML document, visit www.w3.org for details) or the DOM whereas with the second approach the database should be accessible via standard technologies such as JDBC. The important thing to a level coursework Clifton College, understand about reading critical thinking city, university of london native XML databases is that they work with the internal structures of the XML documents – they don’t just store them as a binary large object (BLOB) in the database. a level coursework Clifton College! Flat files . Flat files, such as .TXT or. a level coursework Clifton College! CSV (comma separated value) files, are commonly used to essays to write about university of central florida, store data. A single file can be used to store one type of a level coursework Clifton College data structure, such as customer information or sales transaction information, or through a coding and formatting strategy the structures of several types of data structures. One approach to flat file organization is reading critical thinking city, university of london either to have data values separated by a pre-defined character, such as a comma, or tag, such as </FirstName> in an XML document. Introduction to argumentative essay International College of Management (ICMS)! Another common approach is to delimit data values by size – the first 20 characters of the row represent the first name of the customer, the a level coursework Clifton College, next 20 characters represent the surname, and so on. Hierarchical databases . a level coursework Clifton College! Hierarchical databases link data structures together like a family tree such that each record type has only one owner, for example an order is a level coursework Clifton College owned by only one customer. Hierarchical structures were widely used in the first mainframe database management systems, and a level coursework Clifton College, are still a very common source of data in many large organizations. Hierarchical databases fell out of a level coursework Clifton College favor with the essays to write about university of central florida, advent of relational databases due to short journal articles abingdon school, their lack of flexibility because it wouldn’t easily support data access outside the where can i pay someone to write my essay anglia ruskin university (navitas), original design of the data structure. For example, in the customer-order schema you could only access an order through a customer, you couldn’t easily find all the orders that included the sale of short journal articles abingdon school a widget because the schema isn’t designed to all that. Prevalence layer . Klaus Wuestefeld defines prevalence as “transparent persistence, fault-tolerance and load-balancing of the execution of the business logic of an information system through the use of state snapshots as well as command and query queuing or logging”. A prevalence layer is effectively a simple persistence framework that serializes objects and writes them to essays to write about university of central florida, log files. From the point of view of where can i pay someone to write my essay anglia ruskin university (navitas) developers all objects are cached in memory and the persistence of the objects is truly treated as a background task that the developers don’t need to worry about. Table 2 presents a comparison of the various types of a level coursework Clifton College persistence mechanism and notes on essay writing seneca college, provides references to vendors where applicable. Short journal articles Abingdon School! Table 3 presents suggestions for when you might use each type of technology. Large organizations will find that they are using several types of where can i pay someone to write my essay anglia ruskin university (navitas) persistence mechanism and will even install the products of a level coursework Clifton College several different vendors. Not only do you have a choice you might be forced to short journal articles abingdon school, work with a wide range of databases whether you want to or not.