Teach Yourself Borland C++ in 14 Days - portal

C++Builder Database Architecture
A good application of cached updates is in data-entry forms. There are three basic types of
database forms: decision-support forms, transaction-processing forms, and data-entry forms.
Because users of data-entry forms will typically add several rows in succession, it makes sense
to cache these additions locally and then save them in one pass. This will reduce table locking
on your database and speed up the application.
Updating read-only TDatasets is covered in the discussion of the UpdateSQL component later
today.
On...Error
The OnEditError, OnDeleteError, and OnPostError events allow you to react to errors that
occur while modifying the data in a TDataset. These events all send the same three parameters
to handlers you define for them: the TDataset in which the error occurred, the exception class
raised by the error, and a var parameter that lets you specify what action to take once the
handler finishes. Here’s a sample of the method handler that C++Builder generates for the
On...Error events:
void __fastcall TForm1::Table1DeleteError(TDataset *TDataset,
EDatabaseError *E, TDataAction &Action)
{
}
You can set Action to one of three values: daFail, daAbort, or daRetry.
TQuery
Module: DBTables Class ancestor: TDBDataSet
Like TTable, TQuery is a direct descendant of the DBDataSet class and an indirect descendant
of the TDataset class. You use TQuery to send explicit SQL statements to the database engine.
This SQL either operates on local tables or is passed directly to your database server. You
execute a query that returns a result set using TQuery’s Open method or by setting its Active
property to true. Provided that the query adheres to C++Builder’s restrictions on “live”
queries, you can then treat the result set as if it were a table, which is similar to the way a SQL
VIEW works on many database servers. You can update, add to, and delete the rows in this live
result set, just as you can when using a TTable component.
Table 12.11 lists the key properties, Table 12.12 lists the key methods, and Table 12.13 lists
the key events for the TQuery component.
451
12