How to do UNION in flexible query

I want to merge catetegory_id and subCategory_id into one column. But I don't know how to do it in flexible query. Below code is what I want to do

SELECT * FROM 

(SELECT
    ccr.targetpk as pk
    FROM
    Categories cc
    JOIN Cat2CatRel ccr ON cc.pk = ccr.sourcepk
    WHERE cc.p_code = '0106') 

UNION  SELECT CATE.pk as pk FROM CATEGORIES cate WHERE CATE.p_code = '0106'

I need a flexible query which works same as above code.