Generación de consulta SQL basada en parámetros de URL

Hay varias formas de hacer esto, pero la forma más fácil sería recorrer las columnas aceptables y luego agregarlas apropiadamente.

// I generally use array and implode to do list concatenations. It avoids
// the need for a test condition and concatenation. It is debatable as to
// whether this is a faster design, but it is easier and chances are you 
// won't really need to optimize that much over a database table (a table
// with over 10 columns generally needs to be re-thought)
$search = array();
// you want to white-list here. It is safer and it is more likely to prevent
// destructive user error.
$valid  = array( 'condition', 'brand' /* and so on */ );


foreach( $valid as $column )
{
   // does the key exist?
   if( isset( $_GET[ $column ] ) )
   {
      // add it to the search array.
      $search[] = $column . ' = ' . mysql_real_escape_string( $_GET[ $column ] );
   }
}
$sql = 'SELECT * FROM TABLE_NAME WHERE ' . implode( ' AND ', $search );
// run your search.

Si realmente está tratando de deshacerse de las declaraciones 'if', podría usar esto:

$columns = array_intersect( $valid, array_keys( $_GET ) );
foreach( $columns as $column )
{
    $search[] = $column . ' = ' . mysql_real_escape_string( $_GET[ $column ] );
}
$sql = 'SELECT * FROM TABLE_NAME WHERE ' . implode( ' AND ', $search );

Pero es posible que desee ejecutar puntos de referencia reales para determinar si esa es una opción sustancialmente mejor.