Applied Mathematics (mentioned on other responses) is very much worth the read, but it doesn't really go into the details of relational algebra. It does not explain all the nitty gritty details, or even provides any kind of formal definition, of the relational operators. In fact, Applied Mathematics sort of takes that knowledge as prerequisite.
Codd's papers are obviously relevant, but there's two caveats : (1) Codd was a "mathematician pur sang". His writings are therefore rather difficult to grasp for lots of people. (2) His papers date back a few decades, and it's not like the insights and knowledge about the relational model have not evolved during those decades.
The wikipedia article is fairly good, but is imo flawed in two respects : (1) it pretends that outer joins and nulls can be part of the relational model / relational algebra, and (2) it seems to pretend that "the" relational algebra cannot support transitive closure (the correct state of affairs is that both a simple and a generalized transitive closure operator have indeed been defined for relations.).
Judging by what little detail you provided in the question about "what it REALLY is that you're after", I'd say "Introduction to database systems" is a good proposition, as are probably most of Date's books. The problem with that book is that it's a 1000 pages that are intended to cover EVERY aspect of the relational model in an introductory way, which is both more and less than what you seem to be after.
There is a (very) small chapter entitled "introduction to the RA" on my own website http://shark.armchair.mb.ca/~erwin. I've written that chapter from the perspective of "how would I try to explain the RA to someone who doesn't want to go through those 1000 pages, but yet wants to be given accurate information.". The info there does not, I believe, deviate from what Date says. It might be worth a cursory inspection.
"A link I just found to some short video lectures on database theory including one on relational algebra."
You gave the impression that you were interested in genuine mathematical foundations. If that is so, then don't trust anything that contains any of the words "ER", "mapping", ".NET", "microsoft", "SQL", "linq", "object". If that is not so, then please disregard my answer.
'Fundamentals Of Database Systems', Elmasri Navathe, is the book I used at college. That was over 10 years ago, but the fundamentals don't change much. It's quite difficult going, but it's in-depth and comprehensive. You can download it as a free ebook:
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我建议从维基百科页面开始： http://en.wikipedia.org/wiki/Relational_algebra。
让我挖一下我的图书馆，看看能不能找到我学习时用过的书名......
I would suggest starting from the Wikipedia page: http://en.wikipedia.org/wiki/Relational_algebra.
Let me dig my library to see if I can find the name of the books I used when learning it....
应用数学（在其他回复中提到）非常值得一读，但它并没有真正深入关系代数的细节。它没有解释关系运算符的所有细节，甚至没有提供任何类型的正式定义。事实上，应用数学有点以这些知识为先决条件。
科德的论文显然是相关的，但有两个警告：（1）科德是一位“纯粹的数学家”。因此，他的著作对于很多人来说相当难以理解。 (2) 他的论文可以追溯到几十年前，关于关系模型的见解和知识在这几十年里并不是没有发展。
维基百科的文章相当不错，但在我看来有两个方面的缺陷：（1）它假装外连接和空值可以是关系模型/关系代数的一部分，（2）它似乎假装“该”关系代数不能支持传递闭包（正确的情况是，确实为关系定义了简单传递闭包运算符和广义传递闭包运算符。）。
从您在“您真正想要什么”的问题中提供的小细节来看，我认为“数据库系统简介”是一个很好的命题，就像大多数 Date 的书一样。那本书的问题在于，它有 1000 页，旨在以介绍性的方式涵盖关系模型的各个方面，这比您所追求的内容既多又少。
我自己的网站上有一个（非常）小章节，标题为“RA 简介” http://shark .armchair.mb.ca/~erwin。我从“我如何向那些不想阅读这 1000 页，但又想获得准确信息的人解释 RA”的角度来写这一章。我相信，那里的信息与达特所说的并没有什么不同。也许值得粗略检查一下。
Applied Mathematics (mentioned on other responses) is very much worth the read, but it doesn't really go into the details of relational algebra. It does not explain all the nitty gritty details, or even provides any kind of formal definition, of the relational operators. In fact, Applied Mathematics sort of takes that knowledge as prerequisite.
Codd's papers are obviously relevant, but there's two caveats : (1) Codd was a "mathematician pur sang". His writings are therefore rather difficult to grasp for lots of people. (2) His papers date back a few decades, and it's not like the insights and knowledge about the relational model have not evolved during those decades.
The wikipedia article is fairly good, but is imo flawed in two respects : (1) it pretends that outer joins and nulls can be part of the relational model / relational algebra, and (2) it seems to pretend that "the" relational algebra cannot support transitive closure (the correct state of affairs is that both a simple and a generalized transitive closure operator have indeed been defined for relations.).
Judging by what little detail you provided in the question about "what it REALLY is that you're after", I'd say "Introduction to database systems" is a good proposition, as are probably most of Date's books. The problem with that book is that it's a 1000 pages that are intended to cover EVERY aspect of the relational model in an introductory way, which is both more and less than what you seem to be after.
There is a (very) small chapter entitled "introduction to the RA" on my own website http://shark.armchair.mb.ca/~erwin. I've written that chapter from the perspective of "how would I try to explain the RA to someone who doesn't want to go through those 1000 pages, but yet wants to be given accurate information.". The info there does not, I believe, deviate from what Date says. It might be worth a cursory inspection.
您需要阅读Ed Codd 的论文 。
You'll want to read Ed Codd's papers.
“我刚刚找到了一些关于数据库理论的短视频讲座的链接，其中包括一个关于关系代数的视频讲座。”
你给人的印象是你对真正的数学基础感兴趣。如果是这样，那么不要信任任何包含“ER”、“mapping”、“.NET”、“microsoft”、“SQL”、“linq”、“object”等单词的内容。如果不是这样，请忽略我的回答。
"A link I just found to some short video lectures on database theory including one on relational algebra."
You gave the impression that you were interested in genuine mathematical foundations. If that is so, then don't trust anything that contains any of the words "ER", "mapping", ".NET", "microsoft", "SQL", "linq", "object". If that is not so, then please disregard my answer.
维基百科中的关系代数文章怎么样？
What about the Relational algebra article in Wikipedia?
数据库专业人员应用数学，作者：Lex de Haan 和 Toon Koppelaars
Applied Mathematics for Database Professionals by Lex de Haan and Toon Koppelaars
Chris Date 的“数据库系统简介”是一本经典书籍。还有youtube.com/edu 上有关数据库的讲座视频
Chris Date's "An Introduction to Database Systems" is a classical book. There are also videos of lectures on databases on youtube.com/edu
《数据库系统基础》，Elmasri Navathe，是我在大学时使用的书。那是十多年前的事了，但基本面没有太大变化。虽然很难，但是很深入、很全面。您可以将其作为免费电子书下载：
http://www.freeebooks4you.co.cc /2009/09/fundamentals-of-database-systems.html
第 7 章介绍关系代数。
'Fundamentals Of Database Systems', Elmasri Navathe, is the book I used at college. That was over 10 years ago, but the fundamentals don't change much. It's quite difficult going, but it's in-depth and comprehensive. You can download it as a free ebook:
http://www.freeebooks4you.co.cc/2009/09/fundamentals-of-database-systems.html
Chapter 7 covers relational algebra.