Databases get a grip on XML

If you could do one thing to improve integration and automate processes with customers and business partners, it would be to implement XML, which has become the standard for exchanging information between disparate systems because it is easily transformed into any format. The good news is that the four leading relational databases, namely Oracle Database, IBM DB2, Sybase ASE, and Microsoft SQL Server, not only can store XML data, but they hide much of the complexity of working with XML.

What does a fashionable XML database provide? Four basic functions: the ability to consume, store, search, and generate XML. The extent to which the database supports these functions and the methods it uses to accomplish them are what make for a successful implementation of XML in a database.

Relational databases and XML documents are both powerful ways to represent relationships among data, but they're powerful in different ways. For example, querying on a patient ID number in a relational database may allow you to quickly find the dates a certain patient visited the hospital, the conditions he was diagnosed with, and the treatments he was given. But it likely won't help you determine which treatments were provided for which conditions or what times the treatments took place, nor will it give you other useful information that XML versions of these records could provide.

But whether or not you can combine the benefits of relational and XML data depends on how you store the XML. There are three methods for physically storing XML data in a relational database: shredded, unstructured, and structured. Shredded and unstructured are useful methods but limited. The structured method allows you to leverage the power of both relational data and XML hierarchies.

Shredding puts XML data into relational columns but strips it of its XMLness, meaning the hierarchical relationships among the data in the original XML document are lost. Shredding is useful when you're not concerned about keeping the data in XML format. For example, let's say you have a Web site that allows customers to place orders, and the order needs to go to a number of different database systems. Producing an XML file and having the different systems pick it up——that is, shred it——from a network share may be the most efficient and error-free way to get the data where you want it to go.

The unstructured method uses a data type called a CLOB (Character Large Object) to store an entire XML document as a single unit. Databases have been doing this for years with different types of documents, so this is nothing new. The unstructured method provides limited search capabilities, but it is still quite useful. You can't base queries on it, but the structure of the original data is preserved. A good use for unstructured XML storage would be in keeping original documents to comply with government regulations. For example, if a financial institution were to receive original loan documents in XML, this would allow them to have a relational record of each loan application, and also to store the original application with that record.

The structured method allows you to store XML data inside the database and preserve the hierarchy of the data. Structured storage, also known as “native XML”storage, is what every vendor is trying to achieve. The most obvious benefit of preserving the hierarchical relationships of XML data is being able to receive an XML document, combine it or manipulate it with relational data, and produce XML as a result. It isn't possible to produce such result sets with a relational query language alone.

時(shí)文選讀

數(shù)據(jù)庫(kù)抓住XML

如果你能做件事來(lái)改進(jìn)(應(yīng)用)集成、實(shí)現(xiàn)與客戶和商務(wù)伙伴合作的自動(dòng)化，那它就是實(shí)施XML，XML已經(jīng)成為不同系統(tǒng)之間交換信息的標(biāo)準(zhǔn)，因?yàn)樗苋菀邹D(zhuǎn)換成任何格式。令人高興的是，四個(gè)主要的關(guān)系數(shù)據(jù)庫(kù)系統(tǒng)，即Oracle、IBM DB2、Sybase ASE和Microsoft SQL Server，不僅能儲(chǔ)存XML數(shù)據(jù)，而且還隱藏掉使用XML時(shí)的很多復(fù)雜性。

那么時(shí)髦的XML數(shù)據(jù)庫(kù)能提供哪些功能?有四項(xiàng)基本功能：消費(fèi)、儲(chǔ)存、搜索和生成XML。在多大的程度上支持這些功能和實(shí)現(xiàn)這些功能所使用的方法，成為在數(shù)據(jù)庫(kù)中實(shí)施XML的關(guān)鍵。

關(guān)系數(shù)據(jù)庫(kù)和XML文檔都是表示數(shù)據(jù)之間關(guān)系的重要方法，但是它們的重要性表現(xiàn)在不同的地方。例如，查詢關(guān)系數(shù)據(jù)庫(kù)中病人的身份證號(hào)碼，可以讓你快速發(fā)現(xiàn)某個(gè)病人到醫(yī)院看病的日期、診斷的病情和接受的治療。但此數(shù)據(jù)庫(kù)不可能幫你確定對(duì)哪種病情提供了哪種治療、或者治療了多少次，也不能提供其他的有用信息，而這些病歷的XML版本能提供這些信息。

你能否合并關(guān)系數(shù)據(jù)和XML數(shù)據(jù)兩者的長(zhǎng)處，依賴于你如何存儲(chǔ)數(shù)據(jù)。在關(guān)系數(shù)據(jù)庫(kù)中物理地存儲(chǔ)XML數(shù)據(jù)有三種方法：切碎、非結(jié)構(gòu)化和結(jié)構(gòu)化。切碎和非結(jié)構(gòu)化是有用的方法，但有局限性。而結(jié)構(gòu)化方法讓你可以利用關(guān)系數(shù)據(jù)和XML層次結(jié)構(gòu)兩者的力量。

切碎是將XML數(shù)據(jù)放進(jìn)關(guān)系列中，但去掉了它的XML特征，這意味著原來(lái)XML文檔中數(shù)據(jù)的層次關(guān)系丟失了。當(dāng)你對(duì)是否按XML格式保存數(shù)據(jù)無(wú)所謂時(shí)，切碎法是有用的。例如，你有一網(wǎng)站，允許客戶下訂單，訂單要經(jīng)過(guò)多個(gè)不同的數(shù)據(jù)庫(kù)系統(tǒng)。產(chǎn)生一個(gè)XML文件，從一個(gè)共享網(wǎng)絡(luò)上讓不同的系統(tǒng)選取——就是將它切碎，這可能就是最有效、沒有錯(cuò)誤的方法，讓數(shù)據(jù)到你想讓它到的地方去。

非結(jié)構(gòu)化方法使用了一個(gè)叫CLOB(字符型大對(duì)象)的數(shù)據(jù)類型，將整個(gè)XML文檔作為單個(gè)單元存儲(chǔ)起來(lái)。數(shù)據(jù)庫(kù)利用此方法處理不同類型的文檔已有多年了，因此它不是新東西。非結(jié)構(gòu)化方法提供了有限的搜索功能，但它還是很好用。你不能將查詢建在此基礎(chǔ)上，但初始數(shù)據(jù)的結(jié)構(gòu)保留了下來(lái)。非結(jié)構(gòu)化存儲(chǔ)的一項(xiàng)很好的用途，就是保存原始文檔，使之符合政府規(guī)章。比如，一家金融機(jī)構(gòu)計(jì)劃接受采用XML格式的原始貸款文檔，那么這就允許他們對(duì)每個(gè)貸款申請(qǐng)都有一個(gè)關(guān)系紀(jì)錄，同時(shí)保存著有此紀(jì)錄的原始申請(qǐng)。

結(jié)構(gòu)化方法允許你在數(shù)據(jù)庫(kù)中存儲(chǔ)XML數(shù)據(jù)和保留數(shù)據(jù)的層次關(guān)系。結(jié)構(gòu)化存儲(chǔ)也叫“原始XML”儲(chǔ)存，所有的供應(yīng)商都想實(shí)現(xiàn)它。保留XML數(shù)據(jù)的層次關(guān)系最明顯的好處，就是能接收XML文檔，將它與關(guān)系數(shù)據(jù)合并或進(jìn)行操作，并最終產(chǎn)生XML。單獨(dú)用關(guān)系查詢語(yǔ)言是不能產(chǎn)生這樣的結(jié)果的。