Help:Property constraints portal
Property constraints are rules on properties that specify how properties should be used. The Wikidata model itself is very flexible: nothing stops you from adding Universe (Q1) as head of government (P6). However, a constraint on the property can tell you that a head of government (P6) is usually a human.
Constraints are hints, not firm restrictions, and are meant as a help or guidance to the editor. They can have exceptions: for example, the town of Talkeetna (Q668224) elected the cat Stubbs (Q7627362) as mayor.
Some constraint types, currently one-of constraint and allowed qualifiers constraint, are also used to provide better suggestions when editing statements.
Popular constraints
Constraint types alphabetically
The following types of constraints are available:
- Commons link constraint
- Values for this statement should be valid links to Wikimedia Commons. Examples: image (P18) (“File” namespace), Commons category (P373) (“Category” namespace).
- allowed qualifiers
- See allowed qualifiers constraint.
- allowed qualifiers constraint
- Statements for this property should not have any qualifiers other than the listed ones. Examples: continent (P30) with qualifiers start time (P580) and end time (P582), and Freebase ID (P646) with no qualifiers at all.
- allowed units
- See allowed units constraint.
- allowed units constraint
- Values for this statement should only use certain units (or none). Examples: term length of office (P2097) (unit: year (Q577)), population (P1082) (no units).
- allowed-entity-types constraint
- This property should only be used on certain entity types. Example: Wikidata property example (P1855) (properties).
- citation-needed constraint
- Statements for this property should have at least one reference. Example: properties likely to be challenged.
- conflicts-with constraint
- Items with this property should not have certain other properties or statements. Examples: sex or gender (P21) conflicts with author (P50), which in turn conflicts with instance of (P31)human (Q5).
- contemporary constraint
- Items with this property and values for this property should coexist at some point in time. Examples: mother (P25), country of citizenship (P27).
- difference-within-range constraint
- aka. difference within range. The difference between values for this property and for another property should be within a certain range or interval. For example, the difference between date of birth (P569) and date of death (P570) should be between 0 and 150 years.
- distinct values
- See distinct-values constraint.
- distinct-values constraint
- aka. unique value and distinct value. No two items should share a value for this property; each item’s value should be unique across all statements for this property in all of Wikidata. Almost all identifier properties have this constraint, since an identifier (ISBN, freebase ID, …) should uniquely identify an entity. Other examples include flag (P163) and anthem (P85).
- format constraint
- Values for this property should have a certain format. Often used for identifiers (n digits long, starts with a certain letter, consists of digits, …).
- integer constraint
- Values for this property should be integers without decimal places.
- inverse constraint
- Statements using this property should always have an inverse statement. Examples: father (P22)/mother (P25) with child (P40).
- item-requires-statement constraint
- Items using this property should also have certain other statements. For example, items with killed by (P157) should have place of death (P20), date of death (P570) and manner of death (P1196)homicide (Q149086).
- multi-value constraint
- Items should have more than one statement with this property (or none). Rarely used, but examples include cast member (P161), has tense (P3103) and has grammatical mood (P3161).
- no-bounds constraint
- Values for this property should not use upper and lower bounds. This applies to most non-physical properties with quantity data type, for which the concept of uncertainties is not defined.
- none-of constraint
- Values for this property should not be one of a given set of items. Example: instance of (P31) should not be woman (Q467).
- one-of constraint
- Values for this property should be one of a given set of items. Example: driving side (P1622) should be either left (Q13196750) or right (Q14565199).
- property scope constraint
- This property should only be used for certain scopes (main value of a statement, in a reference, as qualifier).
- range constraint
- Values for this property should be within a certain range or interval. Examples: flattening (P1102) is always between 0 and 1, and UTC date of spacecraft launch (P619) should not be before .
- required qualifier constraint
- Statements for this property must have all of the listed qualifiers. Example: stock exchange (P414) with ticker symbol (P249) and start time (P580).
- single-best-value constraint
- Items should have one “best” statement with this property. Often used for properties that can change over time: for example, cities and countries usually have a single current head of government (P6).
- single-value constraint
- Items should have no more than one statement with this property. Often used for identifiers, but also many other properties: for example, people generally only have a single place of birth (P19) and place of death (P20).
- subject type constraint
- Items with this property should have a certain type. Example: Items with date of birth (P569) should be instance of (P31) human (Q5) or animal (Q729).
- symmetric constraint
- Statements using this property should always exist in both directions. Examples: sibling (P3373), twinned administrative body (P190).
- value requires claim
- See target required claim
- value-requires-statement constraint
- aka. target required statement. Values for this property should also have certain other statements. For example, screenwriter (P58) values should have sex or gender (P21) and occupation (P106)screenwriter (Q28389).
- value-type constraint
- Values for this property should have a certain type. Example: mother (P25) should be instance of (P31) human (Q5) or animal (Q729).
Several constraints have their own predefined templates and are listed at Help:Property constraints/list of constraints.
Usage instructions
Constraint reports are shown for all logged in users when they visit an entity page.
You can also check all constraints for an item on Special:ConstraintReport, and see a summary of violations across Wikidata on Wikidata:Database reports/Constraint violations.
Constraint type items
There is an item for each constraint type, e. g. subject type constraint (Q21503250).
Constraint statements on properties
Constraints for a property are specified as statements on the property, using property constraint (P2302) and the constraint type item. For example: sibling (P3373)property constraint (P2302)symmetric constraint (Q21510862). Deprecated statements are ignored, so changing a constraint statement’s rank to “deprecated” is a convenient way to temporarily disable a constraint without completely removing it.
Constraint parameters
Many constraints are configurable: one of has a list of the permitted values, type contains the type, etc. These settings which configure the constraint are called parameters.
Constraint parameters are specified as qualifiers on the statement. For example:
⟨ subject type constraint (Q21503250) 
⟩relation (P2309)
⟨ instance of (Q21503252) ⟩class (P2308)
⟨ human (Q5) ⟩There are several general parameters that can be added to any constraint statement:
- constraint status (P2316)
- With the value mandatory constraint (Q21502408), this parameter can be used to mark a constraint as mandatory. Violations of mandatory constraints are more severe than violations of non-mandatory ones.
- exception to constraint (P2303)
- Lists known exceptions to the constraint. On the items listed under this parameter, the constraint is not checked.
- group by (P2304)
- Contains a property by the values of which constraint violations should be grouped. That is, when the database reports list violations for a constraint (for all items), the items with violations are grouped by the value they have for the property given in the group by (P2304) parameter. (The constraints extension does not use this parameter.)
- constraint scope (P4680)
- Specifies in which part of a statement the constraints applies, using the three special values constraint checked on main value (Q46466787), constraint checked on qualifiers (Q46466783) and constraint checked on references (Q46466805). Use any combination of these values to specify where a constraint should be checked. If the parameter is not specified, the default scope depends on the constraint type. (Note that the database reports only list violations on the main value of a statement.)
Custom constraints
With some knowledge of SPARQL one can make very complex constraints for certain properties, above and beyond the restrictions mentioned above. More information is available at Template:Complex constraint. See also the list of properties using complex constraints.
