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====== Dependencies (Parameter database) ======
Parameter values often have dependencies, such as dependency of atomic element (Iron, Methane, Oxygen), on media (concrete/clay/rock) or location (Paris, Gothenburg, Kampala). From the database point-of-view, the dependencies change the parameter dimension. Kd_soil can be a scalar parameter if only one atomic element is ever considered, a vector if values are available for many elements or a matrix if both atomic elements and different soil types are considered.

{{dependencies_db.png}}

The database allows up to two dependencies to be defined for a parameter. If more dependencies are required the user must either create several definitions (Kd_soil, Kd_bentonite, Kd_backfill) or use tags (Kd soil, Kd bentonite, Kd backfill).

A dependency is described with a name (“Atomic element”) and a listing of indices for the dependency (“Iron”, “Methane” etc).


**Subsets**

A subset is used when only some indices of a dependency apply to a parameter. For example, say that the database contains a dependency named “Contaminant” which lists 50+ different compounds. A parameter is to be defined which is only used for hydrophobic compounds. Using the “Contaminant” index list would unnecessarily create long tables for the parameter values. Instead, a sub-set of “Contaminant” can be defined named “Hydrophobic contaminants” which only includes the hydrophobic compounds of “Contaminant”. In the database notation, the Contaminant dependency would be the parent dependency.


**Mapped index lists**

Sometimes a dependency is naturally related to another dependency. For instance, the dependency Species is related to the dependency Organism. One can therefore document that a given dependency “maps” to another when a dependency is defined. A typical case is between atomic element and isotope. In the database notation it is said that the atomic element dependency maps to the isotope dependency. Mapping means that a parameter can be used in both the context of its own dependency and in the context of mapped dependency.

Also see:

  *[[Database design]] 
  *[[Users and roles]]
  *[[Quality assurance]]

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