If a database column with a default value is included, then this value is always displayed in the search inputs and only results with that default value will be displayed in the item list and its impossible to conduct a search, eg:
<?php $this->widget('zii.widgets.grid.CGridView', array(
'id'=>'category-grid',
'dataProvider'=>$model->search(),
'filter'=>$model,
'columns'=>array(
'id',
'category_name',
'sort_order', // <<< this column has a default database value of zero
array(
'class'=>'CButtonColumn',
),
),
)); ?>