Help:Properties and types

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Properties and types are the basic way of entering semantic data in Semantic MediaWiki. Properties can be viewed as «categories for values in wiki pages». They are used by a simple mark-up, similar to the syntax of links in MediaWiki:

[[Property name::property value]]

This statement defines a value for the property of the given property name. The page where this is used will just show the text for value and not the property assignment.

Creating properties

Turning links into properties

Consider the Wikipedia article on Berlin. This article contains many links to other articles, such as «Germany», «European Union», and «United States». However, the link to «Germany» has a special meaning: it was put there since Berlin is the capital of Germany. To make this knowledge available to computer programs, one would like to «tag» the link

[[Germany]]

in the article text, identifying it as a link that describes a «capital property». With Semantic MediaWiki, this is done by putting a property name and :: in front of the link inside the brackets, thus:

[[Is capital of::Germany]]

In the article, this text still is displayed as a simple hyperlink to «Germany». The additional text Is capital of is the name of the property that classifies the link to Germany. As in the case of categories, the name of the property is arbitrary, but users should try to re-use properties that already appear elsewhere.

To simplify this re-use, every property has its own article in the wiki, just as every category has an article. You can see all the properties in use in the wiki with the Special:Properties page. Just as category articles are prefixed with Category:, all property articles are prefixed with Property: to distinguish them from other articles. So you can also also use MediaWiki's Special:Search page to find existing properties. As with categories, a property's article can be empty, but it is strongly recommended to add a description that explains the intent of the property and its proper usage.

There are various ways of adding properties to pages:

What it does What you type
Classify a link with the property "example property."
 Classify a [[Example property::link]] with the property "example property."
Make alternate text appear in place of the link.
Make [[Example property::link|alternate text]] appear in place of the link.
To hide the property from appearing at all, use a space as the alternate text.
To hide the property [[Example property::link| ]] from appearing at all
use a space as the alternate text.

Note: The space after | is necessary. If left away, the MediaWiki pipe trick applies, but rarely with desirable effects. Even if a space is given, SMW will not print anything, which should be the desired result in most cases (to make a space appear in the text, use   as a space symbol).

To make an ordinary link with :: without creating a property, escape the markup with a colon in front, e.g.
The C++ :: operator.
The [[:C++ :: operator]].
To assign one value to multiple properties, add :: between each name,
e.g. link.
e.g. [[Property1::Property2::link]]. 

Turning values in text into properties

There is other useful information in wiki articles besides links to other articles. For example, there is a number in the Berlin article giving its population. To make this knowledge available to computer programs, one would like to "tag" the text

3,396,990 

in the article, identifying it as a value for the "Has population" property. With Semantic MediaWiki, this is done by putting the property name and :: in front of the text and surrounding it with [[ ]] brackets, thus:

[[Has population::3,396,990]].

This works fine. However, it creates a link to a 3,396,990 page, and having an article for every population value probably does not make sense. Furthermore, if you wanted to create a list of all German cities ordered by population, numeric order is different from the alphabetical order that you would expect for article names. For example, in alphabetical order, "1,000,000" comes before "345". We want to be able to tell Semantic MediaWiki that "Has population" is a number, not a link to a page in the wiki. The way to do this is to specify a type for the "Has population" property; see the section on datatypes for more information.

Naming of properties

The naming of properties is more important than one would probably expect at first. Property names should avoid ambiguity and confusion. Thus, it is good practice to create Property names as a verb phrase. Here's an example that uses Berlin and makes this practice more comprehensible:

Germany's capital is Berlin. ↔ Berlin is the capital of Germany.

In both cases one could assign a property named "Capital", but this does not convey exactly the intended meaning. The first sentences says that Germany has a capital called Berlin. Therefore the better name for the property is "Has capital". The second sentence states that Berlin is the capital of Germany. Hence the better name for the property is "Is capital" in this case. So the annotation avoiding ambiguity is as follows:

Germany's capital is [[Has capital::Berlin]]. ↔ [[Is capital::Berlin]] is the capital of Germany.

For technical reasons a property name should not be the same as the name of one of the datatypes. So an annotation like e.g. [[Code:Qsdr-5t7Z-b99N]] will not serve the intended purpose, since the assigned datatype will always be "Code" and cannot be changed to "Text". However, to name the property "Is registration code" or something alike is the better alternative for this in the first place anyway.

Datatypes for properties

Semantic MediaWiki has several built-in datatypes from which we can choose for properties. For our previous population example, the appropriate datatype is Number. We want to assign to property "Has population" a special property that specifies it has the datatype "number". To support this, Semantic MediaWiki has a built-in special property called Has type. We use the same syntax for this special property as for any other property, so on the Property:Population page, we write the following which specifies the datatype of the property:

[[Has type::number]]

Semantic MediaWiki knows a number of special properties like Has type. Regardless of whether these properties have their own articles in the wiki, they have a special built-in meaning and are not evaluated like other properties.

Datatypes are very important for evaluating properties. Firstly, the data type determines how tools should handle the given values, e.g. for displaying values and sorting values in search results. Secondly, the data type is required to understand which values have the same meaning, e.g. the values "1532", "1,532", and "1.532e3" all encode the same number. Finally, some data types have special behavior, as will be described below. For these reasons, every property used should be defined with a data type.

The reason we didn't have to specify a data type for the "Is capital of" property above is that the default data type is Page, which displays as a link. Even though the datatype Page is the default, you should explicitly specify a datatype for every property, just to prevent confusion or later redefinition with an unintended type. It is also possible for the administrator of an SMW website to change the default datatype.

The same mark-up for properties that are links to pages also works for properties of other datatypes. Here are some more examples:

What it does What you type
Assign the value 1,234,567 to the property "Has population".
This country has a population of [[Has population::1,234,567]]."
Assign a numeric value, but showing different text in the article.
This country has a population of [[Has population::999,331|about a million]].
Specifying the type in a property's article, e.g.
This property is a number.
This property is a [[Has type::number]].
Display a web link by using a property of type "URL".
Find us online at [[Has website::http://example.com]].
Display a "mailto" link by using a property of type "Email"
You can reach John at [[Has email address::john@example.com]].

List of datatypes

Using different types, properties can be used to describe very different kinds of values. A complete list of available types is available from Special:Types. The available types are:

PageDatatypeDescriptionProvided by
Type Annotation URIAnnotation URIHolds URIs, but has some technical differences during export compared to the "URL" typeSemantic MediaWiki
Type BooleanBooleanHolds boolean (true/false) valuesSemantic MediaWiki
Type CodeCodeHolds technical, pre-formatted texts (similar to type Text)Semantic MediaWiki
Type DateDateHolds particular points in timeSemantic MediaWiki
Type EmailEmailHolds e-mail addressesSemantic MediaWiki
Type Geographic coordinateGeographic coordinateHolds coordinates describing geographic locationsSemantic Maps
Type NumberNumberHolds integer and decimal numbers, with an optional exponentSemantic MediaWiki
Type PagePageHolds names of wiki pages, and displays them as a linkSemantic MediaWiki
Type QuantityQuantityHolds values that describe quantities, containing both a number and a unitSemantic MediaWiki
Type RecordRecordAllows saving compound property values that consist of a short list of values with fixed type and orderSemantic MediaWiki
Type Telephone numberTelephone numberHolds international telephone numbers based on the RFC 3966 standardSemantic MediaWiki
Type TemperatureTemperatureHolds temperature values (similar to type Quantity)Semantic MediaWiki
Type TextTextHolds text of arbitrary lengthSemantic MediaWiki
Type URLURLHolds URIs, URNs and URLsSemantic MediaWiki

Enumerations and "Allows value"

In addition to assigning a datatype to a property, you can also restrict the allowed values to a given set (like a whitelist for property inputs). This is done by using the special property Property:Allows value to enumerate its permitted values. This works for every datatype.

Units

The datatype Quantity allows a unit after the numeric value to distinguish values (e.g. "30 miles" versus "42 km"). To support automatic conversion and multiple unit formats, one needs to declare each supported unit as described in the documentation on custom units. Property values are then automatically converted between different units, so that users are free to use their preferred unit in each article yet still query and compare with property values in other articles.

Inverse properties

Since SMW 1.5.0, it is possible to invert the direction of a property with datatype Page. See the respective help page for detailed information.

Special properties

We mentioned the special property Property:Has type that you use to define the data type of a property. SMW has other predefined special properties that have special meaning (even if you do not create property pages for them in your wiki). You cannot use these names for your own properties, but since SMW 1.4.0 you can use them in browsing and querying interfaces just like all other properties. For more information, see special properties help and the individual property pages.

Silent annotations using #set

Instead of using the standard double-brackets markup, you can also define semantic data using the #set parser function. This function takes in pairs of property names and values and stores them semantically; but it does not print anything to the screen. An example call would be:

{{#set:Has population=3,396,990|Has country=Germany}}

The #set call is specifically helpful when trying to save a Text value that contains square brackets, such as wiki-links; such brackets often don't work with conventional SMW markup.

Defining recurring events

Another type of complex data is recurring events, which are events that have multiple dates, defined according to a preset rule (such as a weekly meeting). You can define the date values for these using the #set_recurring_event parser function, which, like #set, is "silent" and does not print anything. An example call would be:

{{#set_recurring_event:
property=Has date
|start=January 4, 2010
|end=June 8, 2011
|unit=week
|period=1
|include=March 16, 2010;March 23, 2010
|exclude=March 15, 2010;March 22, 2010}}

You can see Help:Recurring events for more information.

Handling in earlier versions

Up to SMW 1.5.6, datatypes had individual wiki pages in their own namespace called "Type". This was abolished to reduce the number of extra namespaces. You may still find expressions like "Type:Number" in places in this wiki. Both writings are accepted for assigning a datatype to a property.

In very early versions of SMW, properties with datatype Page were known as relations and only those used double colons (::) as the separator between property name and link text. All other properties (numbers, text, etc.) were known as attributes and had to use colon equals (:=) as the separator.

See Upgrading from SMW 0.7 to SMW 1.0 for other changes in SMW 1.0; if you're still using the older version of SMW, see semweb:Help:Annotation (SMW0.7) for documentation of Annotations in version 0.7.


This documentation page applies to all SMW versions from 1.6.0 to the most current version.
Other versions: 1.5.0 – 1.5.61.0 – 1.4.3       Other languages: frruukzh-hans

Help:Properties and types en 1.6.0