Help:Property constraints portal/Single value

This is the documentation page for a particular constraint type. See Help:Property constraints portal for a general overview of property constraints.

It is proposed that this informational page become a policy or guideline. Please see the discussion on its talk page or the project chat.
The following is a proposed Wikidata policy or guideline. The proposal may still be in development, under discussion, or in the process of gathering consensus for adoption.

single-value constraint (Q19474404) specifies that a property generally only has a single value.

For example, many identifiers (ISNI (P213), Freebase ID (P646), …) would generally only have a single value, as well as many non-identifier properties like place of birth (P19) and place of death (P20).

If several identifiers apply to the same item, all should be included, not removed.

If sources list several possible place of birth (P19) and place of death (P20), all values can be included and should not be removed. The most trustworthy value can be tagged with preferred rank, to allow data-reusers to get one value instead of getting multiple values. Statements that are backed up by sources that are believed to be incorrect should be tagged with deprecated rank.

If no constraint scope (P4680) is specified, this constraint is checked everywhere.

A qualifier can be defined as separator (P4155). This allows multiple values when using such qualifiers.

Possible actions

There are several possible ways to address a violation of this constraint:

It’s possible that one of the statements belongs to a different item (e. g., the ISNI (P213) of another person), and should be moved to the other item (which may need to be created).
It’s possible that the item conflates multiple topics. Split the item to multiple items.
It’s possible that one of the statements is incorrect and should be set to deprecated rank (e.g. the place of birth (P19) is known, but frequently get mis-reported with the value present).
It’s possible that the external site have duplicated record for a topic. This should be reported to external site.
If the violation is on a qualifier, it is possible that there should be two (or more) statements with one qualifier each, instead of one statement with two (or more) qualifiers using the same property. For instance, if a person received an award more than once, they should have several award received (P166) statements with one point in time (P585) qualifier each, not one award received (P166) statement with several point in time (P585) qualifiers. Likewise, a reference might need to be split up.
It is possible that a separator (P4155) (or several of them) should be defined for the constraint, to declare that multiple values are acceptable as long as they have certain different qualifiers.
If the constraint specifies a legal separator (P4155) (or a set of them), it is possible that such a qualifier should be added to a statement.
It’s possible that the item is a rare, but legitimate exception to the constraint, and nothing should be done.

Parameters

This constraint has one optional parameter:

separator (P4155): Contains the separators. May contain one or more properties. If specified, multiple statements with the same value do not constitute a violation of this constraint as long as they have different qualifiers for the properties specified here.

Examples

Example 1

No parameter.

property constraint

	single-value constraint	edit

	0 references add reference

add value

Example 2 - separator

With separator (P4155) parameter. For example, the capital (P36) may relocate to different place.

property constraint

single-value constraint

edit

separator	start time
	end time
	determination method or standard

0 references

add reference

add value

Template

When the constraint is set to the property, the following template is automatically displayed in the property talk page. The template is created by Module:Constraints using Template:Constraint, invoked from Template:Property documentation. Translations are defined at Module:i18n/constraints.

Single value: this property generally contains a single value. (Help)

Exceptions are possible as rare values may exist. Exceptions can be specified using exception to constraint (P2303).
List of violations of this constraint: Database reports/Constraint violations/P646#Single value, SPARQL

Lists

Lists of properties which are set single value constraint.

Category:Properties with single value constraints
SPARQL query - click the execute button at the linked page to see the query result.