我在使用flink 1.12.0，在按博客里的例子实现temporal table join
https://flink.apache.org/news/2020/12/10/release-1.12.0.html#table-apisql-support-for-temporal-table-joins-in-sql
构造了类似的表格后，写了类似SQL
-- Event-time temporal table join
SELECT
o.order_id,
o.order_time,
o.amount * r.currency_rate AS amount,
r.currency
FROM orders AS o, latest_rates FOR SYSTEM_TIME AS OF o.order_time r
ON o.currency = r.currency;
上传到flink后，提示
py4j.protocol.Py4JJavaError: An error occurred while calling o2.sqlQuery.
: org.apache.flink.table.api.SqlParserException: SQL parse failed. Encountered "ON" at line 8, column 5.
Was expecting one of:
<EOF>
"EXCEPT" ...
"FETCH" ...
"GROUP" ...
在把最后一行的ON改为WHERE后，这个SQL可以解析过去了，但是执行时报了另外的问题
py4j.protocol.Py4JJavaError: An error occurred while calling o153.executeInsert.
: org.apache.flink.table.api.ValidationException: Currently the join key in Temporal Table Join can not be empty.
at org.apache.flink.table.planner.plan.rules.logical.LogicalCorrelateToJoinFromGeneralTemporalTableRule.onMatch(LogicalCorrelateToJoinFromTemporalTableRule.scala:272)
at org.apache.calcite.plan.AbstractRelOptPlanner.fireRule(AbstractRelOptPlanner.java:333)
at org.apache.calcite.plan.hep.HepPlanner.applyRule(HepPlanner.java:542)
请问要如何正确的写该SQL，是否有关于temporal table join的更详细文档？
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