Selecting Data
One way to select data from a database is to use an
RWDBReader to read a database table (See
Reading Tables: Class RWDBReader.) In SQL terms, this is equivalent to:
SELECT * FROM table-name
This technique works well as long as we want to read
all the rows and
all the columns of a single table. Naturally, your applications must do much more than this very simple form of
SELECT. You need an encapsulation of the relational concepts of projection, restriction, and join, as well as special features like ordering and grouping. You may also want to select things other than simple column values. In SQL these features are supported by the
SELECT statement. In the DB Interface Module, the equivalent is the
RWDBSelector.
Each
RWDBSelector instance is an encapsulated
SELECT statement. Its methods provide your application with explicit control over the
SELECT statement's select list, as well as its
FROM,
ON, HAVING, WHERE, ORDER BY, and
GROUP BY clauses. The set operators
+,
*, and
-, which represent union, intersection, and difference, respectively, may be applied to several
RWDBSelector instances in order to achieve the semantics of the
UNION, INTERSECTION, and
DIFFERENCE operations as defined by standard SQL. Because
RWDBSelector instances may be used as expressions, subqueries are also supported.
Class
RWDBSelector supports the full range of functionality built into the SQL
SELECT statement. This means it has many member functions and operators, which are explained in detail in the
SourcePro API Reference Guide. This chapter covers the basic concepts.
