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The rows of the new table are a subset of Cross join or Cartesian product of the two tables, all possible pairs of rows {X1-Y1, X1-Y2, X1-Y3, X2-Y1, X2-Y2, X2-Y3, X3-Y1, X3-Y2, X3-Y3, ...}. Rather than include all possible combinations, each pair is evaluated according to the given spatial predicate; those for which the predicate is true are ...
The following operations are typically supported in GIS software: Intersect: The output will be of the same dimension as the lower of the inputs: Points * {Points, Lines, Polygons} = Points, Lines * {Lines, Polygons} = Lines. This is often used as a form of spatial join, as it merges the attribute tables of the two layers analogous to a table ...
spatial join, in which the attribute tables of two datasets are combined, with rows being matched based on a desired topological relationship between features in the two datasets, rather than using a stored key as in a normal table join in a relational database. For example, joining the attributes of a schools layer to the table of students ...
It was termed "georelational" because in keeping with the principles of relational databases, the geometry and attributes could be joined by matching each shape with a row in the table using a key, such as the row number or an ID number. [20] The spatial database (also called the object-based model [20]) first appeared in the 1990s.
An inner join (or join) requires each row in the two joined tables to have matching column values, and is a commonly used join operation in applications but should not be assumed to be the best choice in all situations. Inner join creates a new result table by combining column values of two tables (A and B) based upon the join-predicate.
Location information (known by the many names mentioned here) is stored in a geographic information system (GIS). There are also many different types of geodata, including vector files, raster files, geographic databases, web files, and multi-temporal data.
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In few scenarios the tables might also be joined using inputs as the key. Indexes can also be used to improve the efficiency of a join. The joined tables need to be stored on a single instance or a machine to further continue processing. There are multiple schemes that are used to pick a machine where a join would be computed.