mysql 子查询sum失效

代码：

// 在mysql5.6，运行正常，5.7报错
SELECT
sum((select a.num  where a.status  =1) ) as total
FROM
    mes_order_product AS a
    LEFT JOIN mes_order AS b ON a.osn = b.osn

修改后的代码，增加了表明和limit

// 5.6 和5.7运行都正常
SELECT
sum((select a.num  from mes_order_product where a.status  =1 limit 0,1))  as total
FROM
    mes_order_product AS a
    LEFT JOIN mes_order AS b ON a.osn = b.osn