sql優(yōu)化：查詢時(shí)加判斷

1.數(shù)據(jù)表的結(jié)構(gòu)：
訂單ID：orderid,
用戶ID：memberid,
下單日期：order_date

2.需求：需要查詢出2017-04月下單的記錄（orderid,memberid），如果當(dāng)前記錄的用戶有在4月份之前下過單，第三列是否訂購過：是，4月份之前沒有下過單的，第三列是否訂購過：否，

3.已經(jīng)寫好的sql:

SELECT orderid,memberid, '是' AS '是否訂購過' FROM ordersum WHERE order_date >= '2017-04-01' AND order_date < '2017-05-01' AND memberid IN (SELECT memberid FROM ordersum WHERE order_date < '2017-04-01')
UNION
SELECT orderid,memberid, '否' AS '是否訂購過' FROM ordersum WHERE order_date >= '2017-04-01' AND order_date < '2017-05-01' AND memberid  NOT IN (SELECT memberid FROM ordersum WHERE order_date < '2017-04-01')
ORDER BY orderid

可以實(shí)現(xiàn)該需求，只是感覺還有優(yōu)化的空間，請(qǐng)大家?guī)兔纯?，謝謝！