ch16.htm

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record are written to the database when the cursor leaves the record. This is fine
for local databases, but it is not a good solution for client/server databases for
a variety of reasons. Most often you hear people talk about network traffic being
the primary reason for using cached updates. Although it is certainly true that cached
updates help reduce network traffic, the value of cached updates goes far beyond
the issue of network traffic. Let me explain further.</P>
<P>Many client/server databases return a read-only result set as the result of a
query. One advantage of cached updates is that the client can work with a local copy
of a dataset, modify it as needed, and then write the edits to the database all at
one time. This is possible because the database server handles updates, insertions,
and deletions of records from a read-only dataset. A local database has to lock records
when they are being actively edited. When a record is locked, other database users
cannot access the record. Using cached updates reduces the time a record is locked
to a very short period of time.</P>
<P>Another advantage to cached updates is that a user can make several changes to
a dataset and then either <I>commit</I> (apply) all changes or <I>rollback</I> (cancel)
all changes. This a two-edged sword, however, because if something happens to go
wrong on the server when changes are being written to the database, all changes are
lost.</P>
<P>One drawback of cached updates is that several users might be working with the
same record at the same time. It then becomes a race to see who gets the record updated
first. In reality, this problem is reduced somewhat by implementing techniques in
the client application that check whether multiple edits have taken place on a record.
For example, if Joe tries to post an update to a record, the database and/or client
application will notify Joe that Mary has changed the record since Joe initially
retrieved it from the database. Joe will have to refresh his copy of the dataset
to see whether he still needs to modify the record.</P>
<P>
<H3><A NAME="Heading13"></A>The Table Component</H3>
<P>The Table component, represented by the TTable class, provides the quickest and
simplest access to a table. Tables are more than adequate for most single-tier database
applications. Usually, you will use the Table component when dealing with local databases
and the Query component when dealing with SQL database servers.</P>
<P>The TTable class has many properties and methods in addition to those in its ancestor
class, TDataSet. Table 16.4 lists the primary properties of the TTable component
and Table 16.5 lists the primary methods. Remember, these are properties and methods
specific to TTable and do not include those of TTable's ancestor, TDataSet.</P>
<P>For the most part, the properties and methods are very intuitive. By that I mean
that you can usually figure out what a property or method does by just looking at
its name. It doesn't take a lot to figure out that the LockTable method locks a table
for an application's specific use and that the UnlockTable method unlocks the table
again. Likewise, you don't have to have an IQ of 150 to guess what the CreateTable,
DeleteTable, and RenameTable methods do. With that in mind, I'm not going to cover
every aspect of every property and method listed here. Instead, let's get on to some
of the more interesting aspects of the Table component.</P>
<P>
<H4>TABLE 16.4. PRIMARY TTable PROPERTIES.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Property</I></TD>
		<TD ALIGN="LEFT"><I>Description</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Exclusive</TD>
		<TD ALIGN="LEFT">Locks a local table so that only this application can use it.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IndexDefs</TD>
		<TD ALIGN="LEFT">Contains information about the table's indexes.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IndexFieldCount</TD>
		<TD ALIGN="LEFT">The number of fields that make up the current key.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IndexFieldNames</TD>
		<TD ALIGN="LEFT">Used to set the current key by specifying the names of the fields to use for the
			index.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IndexFields</TD>
		<TD ALIGN="LEFT">Used to retrieve information about a specific field in an index.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">IndexName</TD>
		<TD ALIGN="LEFT">Used to specify a secondary index for a table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">KeyFieldCount</TD>
		<TD ALIGN="LEFT">The number of fields to use when searching on partial keys.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">MasterFields</TD>
		<TD ALIGN="LEFT">The field or fields that should join the master and detail tables.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">MasterSource</TD>
		<TD ALIGN="LEFT">The table to be used as a master table when this table is used as a detail table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">ReadOnly</TD>
		<TD ALIGN="LEFT">Specifies whether this table is read-only.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">TableName</TD>
		<TD ALIGN="LEFT">The name of the database table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">TableType</TD>
		<TD ALIGN="LEFT">The table's type (Paradox, dBASE, or ASCII).</TD>
	</TR>
</TABLE>

<H4>Table 16.5. Primary TTABLE methods.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Method</I></TD>
		<TD ALIGN="LEFT"><I>Description</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">AddIndex</TD>
		<TD ALIGN="LEFT">Creates a new index for the table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">ApplyRange</TD>
		<TD ALIGN="LEFT">Applies a range to the dataset. Only records within that the range (determined by
			SetRangeStart and SetRangeEnd) are available for viewing or editing.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">BatchMove</TD>
		<TD ALIGN="LEFT">Moves records from a dataset into the table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CancelRange</TD>
		<TD ALIGN="LEFT">Removes any ranges currently in effect for the table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CreateTable</TD>
		<TD ALIGN="LEFT">Re-creates the table using new information.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">DeleteIndex</TD>
		<TD ALIGN="LEFT">Deletes a secondary index.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">DeleteTable</TD>
		<TD ALIGN="LEFT">Deletes a table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">EmptyTable</TD>
		<TD ALIGN="LEFT">Deletes all records from the table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">GetIndexNames</TD>
		<TD ALIGN="LEFT">Retrieves a list of all indexes for the table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">GotoKey</TD>
		<TD ALIGN="LEFT">Moves the cursor to the record indicated by the current key.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">GotoNearest</TD>
		<TD ALIGN="LEFT">Moves the cursor to the record that most closely matches the current key.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">LockTable</TD>
		<TD ALIGN="LEFT">Locks a table so that other applications cannot access it.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">RenameTable</TD>
		<TD ALIGN="LEFT">Renames the table.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">SetKey</TD>
		<TD ALIGN="LEFT">Enables you to set keys for the dataset.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">SetRange</TD>
		<TD ALIGN="LEFT">Sets the start and end range for a dataset and applies the range. This method performs
			the same action as calling the SetRangeStart, SetRangeEnd, and ApplyRange methods.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">SetRangeEnd</TD>
		<TD ALIGN="LEFT">Sets the end of the range.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">SetRangeStart</TD>
		<TD ALIGN="LEFT">Sets the beginning of the range.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">UnlockTable</TD>
		<TD ALIGN="LEFT">Unlocks a table that was previously locked with LockTable.</TD>
	</TR>
</TABLE>



<BLOCKQUOTE>
	<P>
<HR>
<strong>NOTE:</strong> As you have already seen, the DatabaseName property is used to select
	a BDE alias. For local databases, rather than select an alias from the list, you
	can enter a directory where database files are located. The TableName property will
	then contain a list of database tables in that directory. 
<HR>


</BLOCKQUOTE>

<H4>Filters</H4>
<PRE>A common need of a database application is to filter a table. Before I discuss filters in detail, I want to point out that filters are primarily used on local databases. Filters are rarely used with client/server databases; instead, a SQL query 
is used to achieve the same effect that filters have on local databases.
</PRE>
<P>So why filter? Consider that you might have a table with thousands of records,
but you are interested in displaying or working on only a small subset of the table.
Let's say you have a database that contains names and addresses of computer users
all over the world. Your company sells these names and addresses to other companies
that want to do bulk mailings.</P>
<P>I call and want to order a mailing list from your company, but I want the list
to contain only those computer users who live in Colorado. You could filter your
table by postal code and generate a list of names with only Colorado addresses. Or,
maybe Borland calls you and wants a list of computer users in Great Britain who are
programmers by occupation. In that case, you could filter by occupation and country,
thereby giving only the names and addresses the customer is interested in.</P>
<P><B>Using Filters in the Table Component&#160;&#160;</B>Filters in the Table component
are handled in one of two ways: through the Filter property or the OnFilterRecord
event. Before I discuss these, let me talk about the Filtered property. This property
determines whether the table is filtered. If Filtered is True, the table will apply
the filter currently in force (either the contents of the Filter property or the
results of the OnFilterRecord event). If Filtered is False, the contents of the Filter
property are ignored and the OnFilterRecord event is never generated.</P>
<P>For the Filter property, you implement a field name, a logical operator, and a
value. A filter might look like this:</P>
<P>
<PRE>FirstName = `Bob'
</PRE>
<P>This statement, in effect, says, &quot;Show me all records in which the first
name is Bob.&quot; Filters can also use the keywords AND, OR, or NOT:</P>
<P>
<PRE>CustNo = 1384 AND ShipDate &lt; `1/1/94'
</PRE>


<BLOCKQUOTE>
	<P>
<HR>
<strong>NOTE:</strong> The field name and the logical operators (AND, OR, or NOT) are not
	case sensitive. The following two filter statements are identical: 
<HR>


</BLOCKQUOTE>

<PRE>CustName = `TurboPower' and ShipDate &lt; `1/1/94'
CUSTNAME = `TurboPower' AND SHIPDATE &lt; `1/1/94'
</PRE>
<P>In the case of searching for text, the FilterOptions property determines whether
the search string is interpreted as case sensitive.</P>
<P>The following operators can be used in filter statements:</P>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Operator</I></TD>
		<TD ALIGN="LEFT"><I>Use</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&lt;</TD>
		<TD ALIGN="LEFT">Less than</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&gt;</TD>
		<TD ALIGN="LEFT">Greater than</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">=</TD>
		<TD ALIGN="LEFT">Equal to</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&lt;&gt;</TD>
		<TD ALIGN="LEFT">Not equal to</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&gt;=</TD>
		<TD ALIGN="LEFT">Greater than or equal to</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">&lt;=</TD>
		<TD ALIGN="LEFT">Less than or equal to</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">()</TD>
		<TD ALIGN="LEFT">Used to specify the evaluation order of compound</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">expressions</TD>
		<TD ALIGN="LEFT"></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">[]</TD>
		<TD ALIGN="LEFT">Used around field names containing spaces</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">