DBMS方面的应用中检索是必不可少的操作,通常我们习惯使用SQL来进行检索,就像下面这样:
//SQL keywords definitions
_LIT(KSelect, "SELECT ");
_LIT(KDelete, "DELETE " );
_LIT(KFrom, " FROM ");
_LIT(KWhere, " WHERE ");
_LIT(KEqual, " = ");
_LIT(KNotEqual, " <> ");
_LIT(KOrderBy, " ORDER BY ");
_LIT(KDot, ", ");
_LIT(KAnd, " AND " );
_LIT(KLike, " LIKE ");
_LIT(KQuotation, "'");
_LIT( KAsterisk, "*" );
// Implementation specific constants
const int KCustomSqlMaxLength = 256;
/**
* Function: GetColumnDataByKeyL
*
* Description: Get a value from appointed table and column by key.
*
* Returns: KErrNone or one of the system wide error codes.
*/
TInt COurDb::GetColumnDataByKeyL( const TDesC &aTable, const TDesC &aCol, const TDesC &aSortCol,
const TDesC &aKeyCol, const TInt aKey, TDes &aVal )
{
TBuf<KCustomSqlMaxLength> sqlStr;
sqlStr.Append( KSelect );
sqlStr.Append( aCol );
sqlStr.Append( KFrom );
sqlStr.Append( aTable );
sqlStr.Append( KWhere );
sqlStr.Append( aKeyCol );
sqlStr.Append( KEqual );
sqlStr.AppendNum( aKey );
sqlStr.Append( KOrderBy );
sqlStr.Append( aSortCol );
// Create a view on the database
RDbView view;
TInt ret( 0 );
ret = view.Prepare( iDb, TDbQuery( sqlStr ), view.EReadOnly );
if( ret != KErrNone )
{
view.Close();
return ret;
}
User::LeaveIfError( view.EvaluateAll() );
CDbColSet* colSet = view.ColSetL();
CleanupStack::PushL(colSet);
if( view.FirstL() )
{
ret = KErrNone;
view.GetL();
aVal.Copy( view.ColDes( colSet->ColNo( aCol ) ) );
}
else
{
ret = KErrNotFound;
}
CleanupStack::PopAndDestroy(colSet);
view.Close();
return ret;
}
上面的方法对程序员来说是最得心应手的,但由于需要组织一长串的SQL语句所以略显冗长,其实Symbian DBMS API提供了检索的功能,可以通过RDbTable:: SeekL()进行单一和复合的查询功能,应用RDbTable:: SeekL()上面的代码就可以变成:
/**
* Function: GetColumnDataByKeyL
*
* Description: Get a value from appointed table and column by key.
*
* Returns: KErrNone or one of the system wide error codes.
*/
TInt COurDb::GetColumnDataByKeyL( const TDesC &aTable, const TDesC &aCol, const TDesC &aIndex,
const TInt aKey, TDes &aVal )
{
// Create an read-only database table object
RDbTable table;
TInt ret = table.Open( iDb, aTable, table.EReadOnly );
if( ret != KErrNone )
{
return ret;
}
CDbColSet* colSet = table.ColSetL();
CleanupStack::PushL(colSet);
table.SetIndex( aIndex ); //
if( table.SeekL( TDbSeekKey( aKey ) ) )
{
table.GetL();
aVal .Copy( table.ColDes( colSet->ColNo( aCol ) ) );
ret = KErrNone;
}
else
{
ret = KErrNotFound;
}
CleanupStack::PopAndDestroy( colSet );
table.Close();
return ret;
}
使用RDbTable:: SeekL()我们需要为数据表设置当前使用的索引,然后通过构造一个TDbSeekKey查询对象传给RDbTable:: SeekL()进行实际的检索,这么简单就完成了检索,去掉了繁杂字符串组合操作.下面接着介绍一下通过RDbTable:: SeekL()实现复合查询的方法,可能有人看过TDbSeekKey的定义后会被TDbSeekKey::Add()这个方法迷惑,认为通过它为查询对象TDbSeekKey添加键值就行了,可不幸的是我们会在运行时得到一个panic.正确的方法是使用Symbian DBMS提供的另一个查询类TDbSeekMultiKey<TInt S>,它是一个瘦模板类,构造时传入复合查询时key的个数,然后通过Add方法把每个key添加进行就可以了:
TDbSeekMultiKey<2> key;
key.Add( aKey1 );
key.Add( aKey2 );
table.SetIndex( aIndex ); //
if( table.SeekL( key ) )
{
table.GetL();
aVal .Copy( table.ColDes( colSet->ColNo( aCol ) ) );
ret = KErrNone;
}
else
{
ret = KErrNotFound;
}
有一点需要注意一下:就是在使用RDbTable:: SeekL()进行检索时,当前使用的索引必需包含每个key所在的列.