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0 pointsrandomdata2y ago0 comments

In my experience, the roads lead back to SQL. It deviates from the relational model. It may even be that SQL was successful because it deviated from the relational model. Perhaps the theory doesn't just work?

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int_19h2y ago· 2 in thread

Roads lead back to SQL because it became a de facto industry standard for "relation-like" stuff.

Can you give an example of a query that cannot be expressed well in relational algebra, but can be in SQL because it deviates from that?

randomdataOP2y ago

> Roads lead back to SQL because it became a de facto industry standard for "relation-like" stuff.

But what was in question is why SQL is the standard. Did it take that position because of its deviation? If so, that would suggest the theory doesn't just work. Without actually profiling, I suspect that the deviation allows some real-world optimizations to take place, enabling SQL databases to be faster than something with strict adherence to the theory. That would be a good reason why you might have to choose SQL over a strict alternative.

> Can you give an example of a query that cannot be expressed well in relational algebra

Seems not. CloudFlare blocked the submission, complaining that I was submitting a SQL query, which it thinks is a security concern for some reason...

In lieu, just think about what a relation is and how SQL is not relational. Even some of the simplest select queries you can imagine can demonstrate your request.

int_19h2y ago

It took that position because it was what the first viable RDBMS used, pretty much. Similar to how JavaScript became the standard PL for browsers.

The simplest SQL queries map perfectly to relational algebra, so I'm still unclear as to what you had in mind. The two major deviations that SQL has over strict relational algebra are non-uniqueness of rows in a table, and NULL. The first one rarely comes up in practice, and any bag of non-unique rows can be trivially mapped to a bag of unique tuples simply by adding synthetic IDs to them. And SQL NULL semantics is widely considered to be a mess even by many users of SQL itself. With respect to performance, NULLs can be implemented very cheaply while optimizing their relational equivalent (1:0-or-1 relation) requires a little bit more effort on the DB side, but it's still such a simple pattern that I don't see a problem here.

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