关系代数语法
今天我第一次遇到数据库关系代数问题，我似乎找不到答案。
我有 3 个表，Batch、Channel 和 Market。
Batch 通过外部键（channelID、marketID）连接到 Channel 和 Market。
这种查询的正确表示法是什么：
select * from batch, channel, market
where batch.channelID=channel.channelID AND batch.marketID=market.marketID
谢谢
如果我理解正确，我需要写如下：
π...(Batch⋈ Channel⋈ Market
(batch.channelID=channel.channelID) (batch.marketID=market.marketID)
First time i encounter a relational algebra for databases question today, and i can't seem to find the answer.
I have 3 tables, Batch, Channel and Market.
Batch is connected to Channel and Market by foregein keys (channelID, marketID).
What is the correct notation for a query of this sort:
select * from batch, channel, market
where batch.channelID=channel.channelID AND batch.marketID=market.marketID
Thanks
If i understood correctly, i need to write as follows:
π...(Batch⋈ Channel⋈ Market
(batch.channelID=channel.channelID) (batch.marketID=market.marketID)
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评论（1）
这将是批量⋈通道⋈市场（自然连接是关联和交换的）。
编辑：由于每个连接的属性名称相同（您将
batch.channelID与channel.channelID进行比较），因此您在可以使用自然连接编写查询的情况下，当代表自然连接时，无需在 ⋈ 运算符下方写入 (xxx = yyy)。您可以将其写为 θ-joins，在这种情况下，您的语法几乎是正确的，但我觉得使用 θ-joins 来表示自然连接有点没有抓住重点。您需要做的更改是，由于 θ-join 不具有关联性，因此您需要添加括号：(batch ⋈ channel) ⋈ market
That would be batch ⋈ channel ⋈ market (natural joins are associative and commutative).
EDIT: since the attribute names are the same for each join (you're comparing
batch.channelIDwithchannel.channelID), you're in a situation where the query can be written with natural joins, and there's no need to write (xxx = yyy) beneath the ⋈ operator when it's representing a natural join.You could write it as θ-joins instead, in which case your syntax would be almost correct, but I feel that using θ-joins to represent natural joins kind of misses the point. The change you need to do is that, since θ-joins are not associative, you need to add brackets : (batch ⋈ channel) ⋈ market