As it was mentioned in chapter 24 “Browsing a table”, an arbitrary number of user representations, that is, queries, can be built for one and the same table (or for several joined tables). By analogy, it is possible to build an arbitrary number of EDB queries on the basis of one EDB table or join of several EDB tables. Right after addition the EDB contains only tables, building of queries can be performed on the basis of these tables by ObjectLand tools.
As its was pointed out above, the essential distinction of EDB queries from the queries on the basis of internal tables is the degree of ObjectLand involvement in the process of building the set of records and fields making part of the query. In the case with internal tables ObjectLand forms a query by inspecting all records of the base tables, all necessary records and fields are picked out, the operations of sorting, calculation, replacement by reference tables and so on are executed. When building the EDB query, all this work is performed outside ObjectLand with the help of the database management system or some other program which manages data of the corresponding source. The role of ObjectLand in this case is to create parameters suitable for SQL statement SELECT and send a request with this statement using ODBC tools. The server program of external data source forms the resultant set of records while executing SELECT statement and sends this set to ObjectLand for further display immediately or when the necessity arises.
ObjectLand provides the user with two different methods of building the query (that is, in fact, with two methods of forming SELECT statement):
Building a query by constructive method, that is, using the same method as when queries on the basis of internal tables of ObjectLand are built. In this case ObjectLand automatically forms SELECT statement with parameters which should provide the same resultant set of records that the user requires. The statement SELECT itself is invisible for the user in this case.
Building a query by specifying SELECT statement in SQL language by the user directly.
Constructive method is convenient because it requires from the user neither knowledge of SQL language, nor additional knowledge apart from the ability to build queries using ObjectLand tools. Specifying of SQL statement directly may be convenient for more experienced users who are familiar with the SQL language. This method permits using all the possibilities of the statement SELECT in a more flexible way.
The queries build by constructive method are marked in the navigation windows with the sign 
(the same sign as for the query on the basis of ordinary tables) and there is a special sign 
for the queries built on the basis of the statement SELECT.
 |  |
