A question of Flink SQL aggregation

[Henry Dai]

Hi, 

Let's assume we have two stream, order stream& order detail stream, they are from mysql binlog.

Table `order` schema: id primary key, order_id and order_status

Table `order_detail` schema: id primary key, order_id and quantity

one order item have several order_detail items

if we have following data sequence:

   order

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