<select id="xxx" > 
    select * from xxx
    <dynamic prepend = "limit " > 
        <isNotEqual property="num" compareValue="0" > 
            0,#num# 
        </isNotEqual> 
    </dynamic> 
</select>

<select id="xxx">
    select * from xxx
    <dynamic prepend="limit"> 
        <isNotNull property="rowStart"> 
        <isNotNull property="pageSize"> 
            #rowStart#, #pageSize# 
        </isNotNull> 
        </isNotNull> 
    </dynamic> 
</select>