As it was noted in section “Maps and tables”, the main purpose of tables and queries in the structure of GIS ObjectLand is storing the data about features of some map. It also refers to EDB tables and queries. ObjectLand provides a possibility to link any type of features with one or several tables or queries of EDB. Such a link can be used to obtain table information about selected features, search on the map features linked with selected records and also set theme filter.
Unlike internal cataloged queries of GDB for which it is impossible to set a straight link with feature types (only the link via a root table is possible), GIS ObjectLand permits a straight link between cataloged queries of EDB (built by constructive method as well as based on specifying SELECT statement directly) with feature types. This possibility is especially important for queries on the basis of SELECT statement, since the root table is not indicated when such queries are created.
The process of creating a link between a feature type and a table (a query) of EDB differs from an analogous operation for an internal table of GDB considerably. First of all one should pay attention to the following: when the link between features and internal GDB tables was discussed in section “Maps and tables”, the nature of this link was not touched upon. From the most general considerations it is clear that in order to establish the link, together with descriptions of map features and/or together with table records the system should store some references which allow passing from the feature to the records linked with it and vice versa. However, ObjectLand user does not need to know where exactly these references are stored. In fact, an internal number of the feature assigned to each feature of GDB map of ObjectLand is used to set a link. However, this number and the whole mechanism of link remain “transparent”, that is, invisible for the user.
A different situation occurs when the link between a feature type and EDB table is set. In this case ObjectLand does not have a possibility to hide from the user fields of a table or a query which provide linking with features, that is why it is necessary to specify explicitly which fields take part in setting the link and what the phrase “the present feature is linked with some particular record” really means. These questions will be described in more detail in next subsections.
One important distinction is that the link between map feature type and internal table of GDB can be either taken into account in the themes built on the basis of this map or not. In this connection it is possible to perform for feature types of theme the operations of creating or removing the link with internal table described in chapter 32 “Links between maps and tables”.
As opposed to that, for links of map feature types with tables and queries of EDB the possibility to refuse from taking into account these links in themes on the basis of the map is not provided. On page “Links” of properties of theme feature type it is possible to view the existing links with EDB components, but it is impossible to remove or change these links.
 |  |
