in & = ?
确定哪些订单购买了 prod_id 为 BR01 的产品（一）
https://www.nowcoder.com/practice/b692b0174c0444fa9452aee2d082fbbb
select cust_id,order_date
from Orders
where order_num in (
select order_num from OrderItems
where prod_id = 'BR01'
);
/*
返回的`order_num`有可能是多个，所以用`in`而非`=`,
尽管有时两者处于某些（单一）条件下，会得到相同的结果,
但它们不是一回事，比如该例中使用会报错,如下:
21000][1242] Subquery returns more than 1 row
*/
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