你可以用 EXISTS :

SELECT t.*
FROM trn t
WHERE EXISTS (SELECT 1 
              FROM cus c 
              WHERE c.sid = 'A0001' AND c.comp_id = t.comp_id AND c.cus_id = t.cus_id
             ) AND
      docdate between '2018-01-01' and '2018-04-30';

你可以用 JOIN ，例如：

SELECT DISTINCT t.*
FROM trn t 
JOIN cus c ON t.comp_id = c.comp_id AND t.cus_id = c.cus_id
WHERE c.sid='A0001' AND t.docdate BETWEEN ('2018-01-01') AND ('2018-04-30');

这个 concat 可能是因为它不能使用索引而导致性能问题。
但是mysql支持多列子查询，所以只需删除它：

SELECT * from trn 
where (comp_id,cus_id) IN 
        ( select comp_id,cus_id
          FROM cus 
          where sid='A0001'
        ) 
  AND docdate between ('2018-01-01') and ('2018-04-30')