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C++Builder Database Architecture
Updates that you make via the DeleteSQL and ModifySQL statements will, of course, need to
be qualified by a SQL WHERE clause. Both of these properties support a special extension to
SQL that enables you to refer to a field’s original value by prefixing its name with Old_. This
is similar to InterBase’s Old. context variable. For example, the ModifySQL statement you set
up for the CUSTOMER table might look like this:
UPDATE CUSTOMER SET Name=:Name
WHERE CustomerNo=:Old_CustomerNo
Though this query doesn’t actually change CustomerNo, it’s a good idea to get into the habit
of using the Old_ prefix anyway, because some updates aren’t possible without it.
TIP
When editing the SQL associated with the TUpdateSQL component, you
can edit your SQL using C++Builder’s full-blown Code Editor. You do
this by clicking the Code Editor button from within the InsertSQL,
DeleteSQL, and ModifySQL property editors. You’ll find C++Builder’s
Code Editor to be much more powerful than the TMemo component
that’s used to edit the three properties by default.
Use your TDataset’s OnUpdateRecord event when you want to perform additional processing
before sending rows to TUpdateSQL. Once you’ve completed this additional processing, you
can call TUpdateSQL’s Apply method to replace the parameters embedded in your SQL and
execute the SQL against your database. Here’s a sample OnUpdateRecord handler:
void __fastcall TForm1::Table1UpdateError(TDataset *TDataset, EDatabaseError *E,
TUpdateKind,TUpdateAction &UpdateAction)
{
#define DEFAULTRATE 5.00
if (UpdateKind == ukInsert)
StoredProc1->FieldByName(“Rate”)->Value=DEFAULTRATE;
UpdateSQL1->Apply(UpdateKind);
UpdateAction=uaApplied;
}
Notice that the routine sets the UpdateAction parameter to tell the cached updates ApplyUpdates
routine that no further action is necessary.
TField
Module: DBTables Class ancestor: TComponent
The TField component is used to access the columns in a TDataset’s rows. Everything that
C++Builder enables you to configure at the field level is done with the TField component.
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