It describes how the differences between data- centric and document- centric documents affect their usage with databases, how XML is commonly used with relational databases, and what native XML databases are and when to use them. NOTE: Although the information discussed in this paper is (mostly) up- to- date, the idea that the world of XML and databases can be seen through the data- centric/document- centric divide is somewhat dated. At the time this paper was originally written (1. XML databases, which were then not widely understood, even in the database community. However, it was always somewhat unrealistic, as many XML documents are not strictly data- centric or document- centric, but somewhere in between. So while the data- centric/document- centric divide is a convenient starting point, it is better to understand the differences between XML- enabled databases and native XML databases and to choose the appropriate database based on your processing needs.

INSERT statements that use VALUES syntax can insert multiple rows. To do this, include multiple lists of column values, each enclosed within parentheses. QuerySet API reference¶ This document describes the details of the QuerySet API. It builds on the material presented in the model and database query guides, so you.

For a more modern look at the difference between XML- enabled and native XML databases, see chapter 1 of XML for DB2 Information Integration. Is XML a Database? Before we start talking about XML and databases, we need to answer a question that occurs to many people: . That is, it is a collection of data. In many ways, this makes it no different from any other file - - after all, all files contain data of some sort. For example, it is self- describing (the markup describes the structure and type names of the data, although not the semantics), it is portable (Unicode), and it can describe data in tree or graph structures.

Back in the post Distributed Joins, Process Skew I mentioned a common cause of process skew were default values such as -1 for an “Unknown” customer creating skew.

Cascading Updates and Deletes, introduced with SQL Server 2000, were such an important, crucial feature that it is hard to imagine providing referential integrity. What is a NULL Value? A field with a NULL value is a field with no value. If a field in a table is optional, it is possible to insert a new record or update a record.

It also has some disadvantages. For example, it is verbose and access to the data is slow due to parsing and text conversion. A more useful question to ask is whether XML and its surrounding technologies constitute a . The answer to this question is, . On the minus side, it lacks many of the things found in real databases: efficient storage, indexes, security, transactions and data integrity, multi- user access, triggers, queries across multiple documents, and so on. Thus, while it may be possible to use an XML document or documents as a database in environments with small amounts of data, few users, and modest performance requirements, this will fail in most production environments, which have many users, strict data integrity requirements, and the need for good performance.

A good example of the type of . It is much easier to invent a small XML language and write a SAX application for interpreting that language than it is to write a parser for comma- delimited files.

In addition, XML allows you to have nested entries, something that is harder to do in comma- delimited files. However, this is hardly a database, since it is read and written linearly, and then only when the application is started and ended. Examples of more sophisticated data sets for which an XML document might be suitable as a database are personal contact lists (names, phone numbers, addresses, etc.), browser bookmarks, and descriptions of the MP3s you've stolen with the help of Napster. However, given the low price and ease of use of databases like d. BASE and Access, there seems little reason to use an XML document as a database even in these cases. The only real advantage of XML is that the data is portable, and this is less of an advantage than it seems due to the widespread availability of tools for serializing databases as XML. Why Use a Database?

The first question you need to ask yourself when you start thinking about XML and databases is why you want to use a database in the first place. Do you have legacy data you want to expose?

Are you looking for a place to store your Web pages? Is the database used by an e- commerce application in which XML is used as a data transport? The answers to these questions will strongly influence your choice of database and middleware (if any), as well as how you use that database. For example, suppose you have an e- commerce application that uses XML as a data transport. It is a good bet that your data has a highly regular structure and is used by non- XML applications. Furthermore, things like entities and the encodings used by XML documents probably aren't important to you - - after all, you are interested in the data, not how it is stored in an XML document. In this case, you'll probably need a relational database and software to transfer the data between XML documents and the database.

If your applications are object- oriented, you might even want a system that can store those objects in the database or serialize them as XML. On the other hand, suppose you have a Web site built from a number of prose- oriented XML documents. Not only do you want to manage the site, you would like to provide a way for users to search its contents. Your documents are likely to have a less regular structure and things such as entity usage are probably important to you because they are a fundamental part of how your documents are structured. In this case, you might want a product like a native XML database or a content management system. This will allow you to preserve physical document structure, support document- level transactions, and execute queries in an XML query language. Data versus Documents.

Perhaps the most important factor in choosing a database is whether you are using the database to store data or documents. For example, is XML used simply as a data transport between the database and a (possibly non- XML) application? Or is its use integral, as in the case of XHTML and Doc. Book documents? This is usually a matter of intent, but it is important because all data- centric documents share a number of characteristics, as do all document- centric documents, and these influence how XML is stored in the database. The next two sections examine these characteristics.(Historical footnote: I first heard the terms data- centric and document- centric on the xml- dev mailing list.

I don't know who coined them, but I've found messages from 1. Data- Centric Documents. Data- centric documents are documents that use XML as a data transport. They are designed for machine consumption and the fact that XML is used at all is usually superfluous.

That is, it is not important to the application or the database that the data is, for some length of time, stored in an XML document. Examples of data- centric documents are sales orders, flight schedules, scientific data, and stock quotes. Nas Wd My Book World Edition Windows. Data- centric documents are characterized by fairly regular structure, fine- grained data (that is, the smallest independent unit of data is at the level of a PCDATA- only element or an attribute), and little or no mixed content. The order in which sibling elements and PCDATA occurs is generally not significant, except when validating the document. Data of the kind that is found in data- centric documents can originate both in the database (in which case you want to expose it as XML) and outside the database (in which case you want to store it in a database). An example of the former is the vast amount of legacy data stored in relational databases; an example of the latter is scientific data gathered by a measurement system and converted to XML. For example, the following sales order document is data- centric.

Sales. Order SONumber=. For example, consider a page on Amazon. Although the page is largely text, the structure of that text is highly regular, much of it is common to all pages describing books, and each piece of page- specific text is limited in size. Thus, the page could be built from a simple, data- centric XML document that contains the information about a single book and is retrieved from the database, and an XSL stylesheet that adds the boilerplate text.

In general, any Web site that dynamically constructs HTML documents today by filling a template with database data can probably be replaced by a series of data- centric XML documents and one or more XSL stylesheets. For example, consider the following document describing a flight.

SQL NULL Values - IS NULL and IS NOT NULLWhat is a NULL Value? A field with a NULL value is a field with no value.

If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this. Then, the field will be saved with a NULL value. Note: It is very important to understand that a NULL value is.

A field with a NULL. How to Test for NULL Values? It is not possible to test for NULL values with comparison operators, such as =, < , or < >.

We will have to use the IS NULL and IS NOT NULL operators instead. IS NULL Syntax. SELECT column. Street. New York. Bloggs. Joe London. Roe. Jane New York. Smith. John. 11. 0 Bishopsgate. London. Suppose that the .

Street. Smith. John.