BD10 Syntax

Use case

• Write-intensive: highly normalized: avoid update anomalies!
• Read-intensive: highly denormalized: avoid joins!

Denormalizing road

• relational database: homogeneous collection of flat items
• document store(semi-structured): heterogeneous collection of arborescent items.

Syntax

One syntax defines a language. If a sentence is well-formed, it belongs to this language. Otherwise, it doesn’t.

XML

Entities

• element, defined by <element-name>...</element-name> or <element-name/>
• attribute:
  • only within the brackets, defined like <a attr="value"/>.
  • And only attributes can appear inside opening element tag.
  • two attributes cannot have the same name within a single element.
• text: only between the element tags, e.g. <a>this is text</a>. cannot appear outside of elements.
• comment: <!-- This is a comment -->
• processing instructoin: defined by <? ...?>,e.g.<?xml vesion="1.0"?>
• CDATA sections: appeared like the text, the format is <![CDATA[ ... ]]>
• Document Type: <!DOCTYPE document[(internal subset)]>
• (Internal) Entity declarations: <!ENTITY name "value">, using &name; to get the value.
• External parsed entities: conditions are relaxed ???: text at top level, multiple elements.. Defined method: <!ENTITY name SYSTEM "path">
• ???External unparsed entities: allows an entity to appear as the attribute’s value??

Entity References

Five: <: <, >: >, ': ‘, ": “, &: &,

Character References

two formats: hex, beginning with &#x and dec, beginning with $#

Norms

XML valid name

• a-z,A-Z, :, _ are allowed anywhere in a name
• 0-9, -, . are allowed but not at start.
• other ASCII characters are not allowed in XML names.

Whitespaces

whitespace matters!
space: #x20;; tabs: #x9;; carriage return: #xD;; newlines: #xA;
Carriage return is automatically replaced with newline before parsing.

if in an element declaring that <item xml:space="preserve">: the space before its sub-elements will not be ignored. By default, such whitespaces are ignored.

Namespace

• prefix: anything, can be omitted.
• local name
• namespace, scope: this and within this element
  using xmlns:prefix-name:namespace to define a prefix, and by prefix:local-name to use QName.
  If the prefix is omitted, it will define a default namespace for this element (and its sub-elements).

If no namespace is defined, all the elements are in no namespace. That means there is no default namespace without definition.

Unprefixed attributes are in no namespace even if there is a default namespace in scope.

Best practice

• put all namespace bindings in root element
• Try to keep the same bindings across all documents whenever possible!
• use parsimoniously

JSON

Elements

• string
• number
• null: Null is invalid.
• boolean: true, false
• array
• object

Well-formedness

Others

XHTML, YAML (the “Python of JSON”), CSV(comma separated values)

Further Reading

XML in a Nutshell Chapter 1, 2, 4, 9

Thanks Alessandro Stolfo for reading notes