In some cases (for instance, if a table linked with a feature type by multiple link is used to define a simple condition of a filter group or when location of the given feature with respect to features of a certain type is checked) additional specification of the condition is necessary. It is possible to illustrate this on an example. Let the feature type “Block” have a multiple link with the table “Building certificates”, as it was in the example of the chapter 32 “Links between maps and tables”, and each block be linked with the records of the table which describe buildings located in this block. In this case it would be incorrect to set the condition: “Show blocks with two-storeyed buildings”. It should be specified how this condition should be interpreted if there are several buildings in the block. The following conditions will be correct, for example: “Blocks which have at least one two-storeyed building” or “Blocks in which all the buildings have two storeys”, or “Blocks in which the average number of storeys in buildings is two”. Such condition specification is performed by choosing relation variant when specifying a simple condition using the table linked by a multiple link.
In the case of using spatial properties of a feature the need to choose a relation variant arises, in particular, on checking distances between features, intersections of features and so on. For instance, it is possible to state, using different variants of the same relation, such conditions as: “The building is located less than 500 meters from one of pollution sources”, “The building is located less than 500 meters from each pollution source” or “Average distance from the building to available pollution sources is less than 500 meters”.
The list of possible relation variants depends on specific relation used.
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