In chapter 30 “Queries” we examined creation of cataloged queries – user representations based on tables which are stored in GDB structure. GIS ObjectLand provides the users with a similar possibility to build EDB queries – user representations based on EDB tables.
The function of EDB queries is just the same as the one of queries from internal tables of GDB: they allow picking up the necessary information from tables and displaying it in the most convenient way. However, forming of the set of records and their fields is carried on quite differently when the query is displayed. For the query built on the basis of internal tables of GDB ObjectLand performs searching in the tables, sorting of relevant records, selecting of displayed fields and so on using its own means. For EDB query, forming of the set of records is performed outside ObjectLand, using other programs which manage the data source. The tasks of ObjectLand include forming the text of SQL statement SELECT containing a request to the data source, sending the request, receiving resultant set of records and displaying it.
ObjectLand supports building of EDB queries of two types:
queries built by constructive method;
queries on the basis of direct specifying of SELECT statement.
EDB queries built by constructive method are created by the user with the help of the same tools as internal queries of GDB. They possess similar properties. Constructive building of the query permits performing the following operations:
Setting the condition of searching records in EDB tables on the basis of which the query is built specifying a list of simple conditions.
Specifying a method of joining EDB tables in the query.
Determining calculated fields.
Setting reference tables for query fields.
Setting condition for sorting records in the query.
Each of user actions described above causes changing of text of SELECT statement and sending a new request with this statement to the data source. The statement SELECT itself remains “transparent”, that is, invisible for the user.
Editability of EDB queries build by constructive method is determined in GIS ObjectLand according to the following rules:
If the query is built by linking EDB tables, the query is accessible only for reading.
If some calculated fields are specified for the query, the query is accessible only for reading.
If it is not allowed to duplicate records for the query, it is accessible only for reading.
In all other cases the possibility to edit the query corresponds to the one of editing the EDB table on the basis of which the query is built.
Queries built on the basis of SELECT statement are created by the user by direct specifying of SELECT statement in SQL language. The rules of forming SQL statement SELECT and the possibilities provided by the statement are fully determined by syntax and semantics of the dialect of SQL language used in database management system on the basis of which the data source was built.
For fields of queries built on the basis of SQL statement Select, GIS ObjectLand permits setting reference tables.
Queries built on the basis of SQL statement SELECT are accessible only for reading.
Similar to queries from internal tables of GDB, EDB queries can be also cataloged in GDB – the owner of the EDB under unique names. One should keep in mind that only description of the query is stored in GDB which provides access to records of EDB tables when opening the query.
If the EDB was not saved, cataloged queries are kept in GDB only during the session of working with GDB.
