<query id="쿼리ID" action="select">
    <tables>
        <table name="document_extra_vars" alias="a" />
        <table name="documents" type="left join" alias="b">
            <conditions>
                <condition operation="equal" column="a.document_srl" default="b.document_srl" />
            </conditions>
        </table>
        <conditions>
            <query operation="in" column="document_srl">
                <tables>
                    <table name="document_extra_vars" alias="a" />
                    <table name="documents" type="left join" alias="b">
                         <conditions>
                             <condition operation="equal" column="a.document_srl" default="b.document_srl" />
                         </conditions>
                 </table>
             </tables>
             <conditions>
                 <condition operation="like" column="a.value" default="???" />
                 <condition operation="like" column="b.title" default="???" pipe="or" />
                 <condition operation="like" column="b.content" default="???" pipe="or" />
            </conditions>
            </query>
        </conditions>
    </tables>
</query>​

테스트 해보진 않았지만... 이런 식으로 하시면 될 것 같습니다.

https://github.com/xpressengine/xe-tutorial/tree/master/XML_Query

XML 쿼리 문법은 위 링크에 대략적으로 나와있습니다.