Help:Properties and types

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.

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:

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 called 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 of" in this case. So the annotation avoiding ambiguity is as follows:

Germany's capital is called Has capital::Berlin. ↔ Is capital of::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 "String". 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:

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:

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:

The #set call is specifically helpful when trying to save a String or 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:

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, strings, etc.) were known as attributes and had to use colon equals (:=</tt>) 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.