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WARNING
C++Builder Database Architecture
Be careful that you don’t make bad assumptions about the Bof and Eof
properties. You can’t assume that Bof will be true just because you’re on
the first row of a table. Nor can you assume that Eof will be true when
you’re on the last row of a table. Typically, an additional Prior or Next
is required to set Bof or Eof to true. For example, the sequence First,
Next, Prior won’t reset Bof to true, but First, Next, Prior, Prior will.
Note that Bof is true immediately after opening a table or calling the
First method, and Eof is true immediately after calling the Last
method.
The Append and Insert methods are used to add blank rows to a TDataset. Append adds a
record to the end of the TDataset, whereas Insert adds it at the current cursor position.
Append and Insert both put the TDataset in dsEdit mode. The AppendRecord and InsertRecord
methods are used to add non-blank rows to a TDataset using a supplied set of field values.
The Delete method deletes the row at the current cursor position. The Edit method allows
modification of rows in the TDataset, placing the TDataset in dsEdit mode. The Post method
saves these changes to the database, whereas Cancel discards them. This is also true of the
Append and Insert methods—you can Post or Cancel them as well.
Local Filters
The Filter, Filtered, and FilterOptions properties facilitate setting up local filters on the
TDataset. Local filtering enables you to filter a TDataset from within the application. This can
be advantageous with TDatasets that have a small number of rows; the entirety of the
TDataset will typically be cached by the BDE anyway, so filtering it locally saves interaction
with the database server or network.
Filter enables you to specify a filter expression for restricting the rows that are visible in the
TDataset. The syntax supported in the expression is similar to that of a SQL WHERE clause.
Fields can be compared to each other and to static values. The operators shown here can be
used to build your filter expressions:
Operator Use
< Less than
> Greater than
>= Greater than or equal to