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write articles online for money La Trobe University (NAVITAS)

For the sake of our discussion a relational database is help me write an essay montverde academy 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 write articles online for money La Trobe University (NAVITAS) modern organizations; the how to write good essay fryeburg academy goal is not to discuss relational theory. write articles online for money La Trobe University (NAVITAS)? RDBMSs are used to store the information required by applications built using procedural technologies such as COBOL or FORTRAN, object technologies such as Java and C#, and component-based technologies such as Visual Basic. Because RDBMSs are the dominant persistent storage technology it is critical that all IT professionals understand at least the basics of RDBMSs, the challenges surrounding the 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 . How to write good essay Fryeburg Academy? To improve access time to a data table you define an index on the table. 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 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 proposed UML data modeling notation. To create a row in the write articles online for money La Trobe University (NAVITAS) Seminar table you would issue an critically thinking glenalmond college, INSERT statement, an example of which is write articles online for money La Trobe University (NAVITAS) shown in critically thinking glenalmond college Code Figure 1. Similarly, Code Figure 2 presents and example of write articles online for money La Trobe University (NAVITAS), how to read a row by issuing a SELECT statement. 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. A very good resource for write articles online for money La Trobe University (NAVITAS), learning SQL is SQL Queries for Mere Mortals by Micheal J. write articles online for money La Trobe University (NAVITAS)? Hernandez and how to write good essay fryeburg academy John L. write articles online for money La Trobe University (NAVITAS)? Viescas. Code Figure 1. How to write a summary about an article Idyllwild Arts Academy? 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. SQL statement to delete data from the "Seminar" table. WHERE SEMINAR_ID > 1701. AND OVERSEER_ID = ‘THX0001138’ Object storage . write articles online for money La Trobe University (NAVITAS)? To store an object in a relational database you need to writing an exploratory essay madeira school, flatten it – create a data representation of the object – because relational databases only store data. 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. How to write a summary about an article Idyllwild Arts Academy? Although storing objects in a relational database sounds like a simple thing to 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 write articles online for money La Trobe University (NAVITAS) relational databases you need to writing an exploratory essay madeira school, learn how to map your object schema to your relational database schema. Implementing behavior within the database . write articles online for money La Trobe University (NAVITAS)? Behavior is implemented in a relational database via stored procedures and/or stored functions that can be invoked internally within the database and often by external applications. Stored functions and procedures are operations that run within an 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 help me write an essay montverde academy, both stored functions and stored procedures. In the past stored procedures were written in 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 how to write good essay fryeburg academy form of write articles online for money La Trobe University (NAVITAS), zero or more records, or a response code, or as a database error message. Critically thinking Glenalmond College? Effective use of stored procedures is discussed in detail in how to write good essay fryeburg academy Implementation Strategies for Persisting Objects in unsw essay writing dulwich college RDBs. Concurrency control . Consider an airline reservation system. Unsw essay writing Dulwich College? 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 click the reservation button at the write articles online for money La Trobe University (NAVITAS) same time. What should happen? If the write articles online for money La Trobe University (NAVITAS) system is working properly only write articles online for money La Trobe University (NAVITAS) 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 write articles online for money La Trobe University (NAVITAS) within your database. Transaction control . write articles online for money La Trobe University (NAVITAS)? A transaction is a collection of actions on your database – such as the saving of, retrieval of, or deletion of unsw essay writing dulwich college, data – which form a work unit. A flat transactions is how to write a summary about an article idyllwild arts academy an “all-or-nothing” approach where all the actions must either succeed or be rolled back (canceled). A nested transaction is an approach where some of the actions are transactions in their own right. How to write good essay Fryeburg Academy? These sub-transactions are committed once successful and are not rolled back if the writing an exploratory essay madeira school 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 critically thinking glenalmond college a critical concept for all developers to understand. Enforcing referential integrity . How to write a summary about an article Idyllwild Arts Academy? Referential integrity (RI) is the assurance that a reference from unsw essay writing dulwich college, one entity to write articles online for money La Trobe University (NAVITAS), another entity is 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. Help me write an essay Montverde Academy? Contrary to popular belief, RI isn’t just a database issue, it’s an issue for your entire system. A customer is implemented as an how to write a summary about an article idyllwild arts academy, object within a Java application and as one or more records in your database – addresses are also implemented as objects and as rows. How to write a summary about an article Idyllwild Arts Academy? To delete an address, you must remove the address object from memory, any direct or indirect references to writing an exploratory essay madeira school, it (an indirect reference to an address would include a customer object knowing the value of the AddressID , the primary key of the write articles online for money La Trobe University (NAVITAS) 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 write articles online for money La Trobe University (NAVITAS), application servers that address object could exist simultaneously on several machines. write articles online for money La Trobe University (NAVITAS)? Furthermore, if you have other applications accessing your database then it is writing an exploratory essay madeira school possible that they too have representations of the address in writing an exploratory essay madeira school their memory as well. Worse yet, if the how to write good essay fryeburg academy address is stored in several places (e.g. different databases) you should also consider taking this into account. All developers should understand basic strategies for how to write good essay fryeburg academy, implementing referential integrity. Table 1 describes the help me write an essay montverde academy 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 SQL query, enabling you to move forward and backward through the result set one or more records at write articles online for money La Trobe University (NAVITAS), 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 write articles online for money La Trobe University (NAVITAS) required because less data is unsw essay writing dulwich college transmitted across the network. Application developers need to understand that the underlying data can change between the how to write good essay fryeburg academy 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 write articles online for money La Trobe University (NAVITAS) result set, or previously retrieved records may have been modified. Critically thinking Glenalmond College? Not all cursors are created equal. Some cursors only allow forward scrolling. Cursors are a resource drain on the 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 write articles online for money La Trobe University (NAVITAS) database. Development of write articles online for money La Trobe University (NAVITAS), data intensive behavior that results in 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 writing an exploratory essay madeira school, 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 write articles online for money La Trobe University (NAVITAS) a procedure that is writing an exploratory essay madeira school 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 critically thinking glenalmond college, 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 write articles online for money La Trobe University (NAVITAS) 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 help me write an essay montverde academy database can easily become a bottleneck if your database doesn’t scale well. Your application source code . 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 coupled to your database schema. Unsw essay writing Dulwich College? This situation is traditionally referred to how to write a 500 word essay about yourself st. louis university (into), 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 your situation because it is very rare in practice. Other application source code . Figure 3 depicts the worst-case scenario for relational databases – a wide variety of software systems are coupled to write articles online for money La Trobe University (NAVITAS), your database schema, a situation that is quite common with existing production databases. write articles online for money La Trobe University (NAVITAS)? It is quite common to find that in how to write a 500 word essay about yourself st. louis university (into) addition to the application that your team is currently working on that other applications, some of which you know about write articles online for money La Trobe University (NAVITAS), and some of which you don’t, are also coupled to your database. Perhaps an online system reads and how to write a summary about an article idyllwild arts academy writes to your database. Perhaps a manager has written a spreadsheet, unbeknownst to write articles online for money La Trobe University (NAVITAS), you, that reads data from writing an exploratory essay madeira school, 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 XML data file or simply so your data can be loaded into unsw essay writing dulwich college another database. Persistence frameworks/layers . Writing an exploratory essay Madeira School? A persistence framework encapsulates the logic for 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. Itself . Coupling exists within your database. A single column is coupled to any stored procedure that references it, other tables that use the column as a foreign key, any view that references the column, and write articles online for money La Trobe University (NAVITAS) so on. Critically thinking Glenalmond College? A simple change could result in several changes throughout your database. Data migration scripts . Changes to your database schema will require changes to write articles online for money La Trobe University (NAVITAS), your data migration scripts. Test code . Testing code includes any source code that puts your database into 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 reflect any database schema changes that you make. Documentation . Critically thinking Glenalmond College? 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 write articles online for money La Trobe University (NAVITAS) descriptive meta data. When your database schema changes the documentation describing will also need to change accordingly. write articles online for money La Trobe University (NAVITAS)? Although Agile Modeling implores you to Update Only When It Hurts, that your documentation doesn’t have to write articles online for money La Trobe University (NAVITAS), 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. write articles online for money La Trobe University (NAVITAS)? To make matters worse the how to write good essay fryeburg academy concept of coupling is virtually ignored within database theory circles. Although most database theory books will cover data normalization in unsw essay writing dulwich college excruciating detail, I argue that normalization is the data community’s way of addressing cohesion. 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 . How to write a 500 word essay about yourself St. Louis University (INTO)? When you are working with a shared database, such as the write articles online for money La Trobe University (NAVITAS) situation implied in how to write a 500 word essay about yourself st. louis university (into) Figure 3, you may find that the performance characteristics of 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 how to write good essay fryeburg academy, 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 Item table is under heavy load and how to write a 500 word essay about yourself st. louis university (into) thus your online application is potentially affected. Data integrity is difficult to ensure with shared databases . Because no single application has control over the data it is very difficult to unsw essay writing dulwich college, be sure that all applications are operating under the same business principles. Unsw essay writing Dulwich College? For example, your application may consider an order as fulfilled once it has been shipped and help me write an essay montverde academy a payment has been received. The original legacy application that is still in use by your customer support representatives to 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 shared databases. Less subtly, imagine what would happen if your online order taking application calculates the total for an order and stores it in the order table, whereas the legacy application calculates the subtotals only for order items but does not total the order. When the order fulfillment application sees an order with no total it calculates the total, and appropriate taxes, whereas if a total already exists it uses the existing figure. If a customer makes an order online and then calls back a few hours later and write articles online for money La Trobe University (NAVITAS) 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 write articles online for money La Trobe University (NAVITAS) 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 write articles online for money La Trobe University (NAVITAS) the other hand, are typically highly denormalized with significant data redundancy within them to support a wide range of write articles online for money La Trobe University (NAVITAS), 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 Persisting Objects in RDBs chapter compares and contrasts various encapsulation strategies that you have available to you. For now assume that it is possible to hide the details of your database schema from the majority of the developers within your organization while at the same time giving them access to write articles online for money La Trobe University (NAVITAS), your database. Some people, often just the Agile DBA(s) responsible for supporting the database, will need to understand and write articles online for money La Trobe University (NAVITAS) work with the underlying schema to maintain and evolve the encapsulation strategy. One advantage of encapsulating access to your database is that it enables application programmers to focus on the 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 write articles online for money La Trobe University (NAVITAS), is keeping the data access classes up to date. This reveals a second advantage to this approach – it provides greater freedom to Agile DBAs to write articles online for money La Trobe University (NAVITAS), do their job. Figure 4 depicts the concept of encapsulating access to your database, showing how the write articles online for money La Trobe University (NAVITAS) best case scenario of Figure 2 and the worst case scenario of Figure 3 would likely change. In the how to write a 500 word essay about yourself st. louis university (into) 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 data-related code would be in writing an exploratory essay madeira school one place, making it easier to modify whenever database schema changes occurred or to support performance-related changes. It’s interesting to 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 write articles online for money La Trobe University (NAVITAS) interface of a data access object changed. write articles online for money La Trobe University (NAVITAS)? You’ll never get away from coupling. However, from how to write good essay fryeburg academy, the point of view of the application programmer this is write articles online for money La Trobe University (NAVITAS) a much easier change to detect and act on – with the database encapsulation strategy in place the application programmers are only dealing with program source code (e.g. Java) and not program source code plus SQL code. Things aren’t quite so ideal for the worst-case scenario. How to write a summary about an article Idyllwild Arts Academy? Although it is write articles online for money La Trobe University (NAVITAS) possible that all applications could take advantage of your encapsulation strategy the reality is that only a subset will be able to. Critically thinking Glenalmond College? Platform incompatibility can be an write articles online for money La Trobe University (NAVITAS), issue – perhaps your data access objects are written in 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 writing an exploratory essay madeira school 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 help me write an essay montverde academy, be made is write articles online for money La Trobe University (NAVITAS) that a portion of help me write an essay montverde academy, your organization’s application will be able to take advantage of your encapsulation strategy and some won’t. Unsw essay writing Dulwich College? There is still a benefit to doing this, you are reducing coupling and write articles online for money La Trobe University (NAVITAS) therefore reducing your development costs and critically thinking glenalmond college maintenance burden, the problem is that it isn’t a fully realized benefit. Another advantage of encapsulating access to a database is write articles online for money La Trobe University (NAVITAS) 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 write articles online for money La Trobe University (NAVITAS) 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 how to write a 500 word essay about yourself st. louis university (into) attributes and behaviors of an address, as well as the ability to navigate objects in addition to an RDBMS’s ability to join tables. Critically thinking Glenalmond College? By implementing objects within the database, an ORDBMS can execute complex analytical and data manipulation operations to search and transform multimedia and unsw essay writing dulwich college other complex objects. ORDBs support the robust transaction and data-management features of RDBMSs while at the same time offer a limited form of the critically thinking glenalmond college flexibility of object-oriented databases. Because of ORDBMSs relational foundation, database administrators work with familiar tabular structures and data definition languages (DDLs) and write articles online for money La Trobe University (NAVITAS) 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. Critically thinking Glenalmond College? They bring much more than persistent storage of write articles online for money La Trobe University (NAVITAS), programming language objects. ODBs extend the write articles online for money La Trobe University (NAVITAS) semantics of how to write good essay fryeburg academy, Java to provide full-featured database programming capability, via new class libraries specific to the ODB vendor, while retaining native language compatibility. A major benefit of this approach is write articles online for money La Trobe University (NAVITAS) the unification of the how to write a 500 word essay about yourself st. louis university (into) application and database development into how to write a 500 word essay about yourself st. louis university (into) a seamless model. As a result, applications require less code, use more natural persistence modeling, and 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 flatten data structures for write articles online for money La Trobe University (NAVITAS), storage, as a result forgoing the marshalling overhead inherent with other persistence mechanism technologies (such as RDBs). write articles online for money La Trobe University (NAVITAS)? This one-to-one mapping of application objects to database objects provides higher performance management of objects and write articles online for money La Trobe University (NAVITAS) enables better management of the complex interrelationships between objects. Native XML databases . 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 help me write an essay montverde academy entire XML document in the file system, perhaps in 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 help me write an essay montverde academy the way they are accessed: with the first approach the only interface to write articles online for money La Trobe University (NAVITAS), the data is XML and related technologies, such as XPath (a language design specifically for critically thinking glenalmond college, addressing parts of an XML document, visit www.w3.org for critically thinking glenalmond college, details) or the DOM whereas with the second approach the database should be accessible via standard technologies such as JDBC. The important thing to understand about native XML databases is write articles online for money La Trobe University (NAVITAS) 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. Flat files . Flat files, such as .TXT or. CSV (comma separated value) files, are commonly used to store data. A single file can be used to critically thinking glenalmond college, store one type of data structure, such as customer information or sales transaction information, or through a coding and formatting strategy the write articles online for money La Trobe University (NAVITAS) structures of several types of data structures. One approach to flat file organization is either to have data values separated by a pre-defined character, such as a comma, or tag, such as </FirstName> in write articles online for money La Trobe University (NAVITAS) an XML document. 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 next 20 characters represent the surname, and so on. Hierarchical databases . Hierarchical databases link data structures together like a family tree such that each record type has only writing an exploratory essay madeira school one owner, for example an order is owned by only one customer. Hierarchical structures were widely used in the first mainframe database management systems, and are still a very common source of data in many large organizations. Hierarchical databases fell out of favor with the advent of relational databases due to their lack of flexibility because it wouldn’t easily support data access outside the original design of the data structure. Unsw essay writing Dulwich College? For example, in the write articles online for money La Trobe University (NAVITAS) customer-order schema you could only help me write an essay montverde academy access an order through a customer, you couldn’t easily find all the how to write good essay fryeburg academy orders that included the sale of 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 how to write good essay fryeburg academy business logic of an information system through the use of write articles online for money La Trobe University (NAVITAS), 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 log files. From the point of view of developers all objects are cached in memory and the persistence of the write articles online for money La Trobe University (NAVITAS) objects is truly treated as a background task that the developers don’t need to worry about. Table 2 presents a comparison of the unsw essay writing dulwich college various types of write articles online for money La Trobe University (NAVITAS), persistence mechanism and provides references to vendors where applicable. How to write a 500 word essay about yourself St. Louis University (INTO)? Table 3 presents suggestions for when you might use each type of technology. Large organizations will find that they are using several types of persistence mechanism and will even install the products of several different vendors. Not only do you have a choice you might be forced to work with a wide range of databases whether you want to or not.