mysql子查询sum失效

// 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

// 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