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Comment by mossmoth | Hacker News Reader

I do not think you are correct in saying that SQL is a logic programming language. A logic programming language is defined in terms of rules/implications[1], which SQL is not. However, it is a declarative language, which Prolog also is. In a declarative language you state what a solution is[2] (which the program then searches for), in contrast to the more traditional type of programming (imperative) that states how to obtain a solution.

[1] - https://en.m.wikipedia.org/wiki/Logic_programming [2] - https://en.m.wikipedia.org/wiki/Declarative_programming

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Well,

If you look at my whole post, I hope it's clear I'm not trying to say SQL is formally anything, just that it's effectively a logic programming language.

IE in an ad-hoc sort-of way SQL can accomplish what a more formal logic programming language accomplish, more or less. You specify a world as a set of tuples and determine whether one set of relations determines another set of relations. That lets you do essentially what prolog can do - for example you can map all the example in the parent article to SQL pretty easily.

> for example you can map all the example in the parent article to SQL pretty easily

How would you map the map coloring example? You can model the color table and the neighbor table in SQL, but that's just the data model. What Prolog also gives you is backtracking search. Is there a good way to do that in SQL? I guess it would work by computing the Cartesian product of states and colors and trying a select on that...

> You specify a world as a set of tuples and determine whether one set of relations determines another set of relations. That lets you do essentially what prolog can do

Datalog, maybe, if you have the search mentioned above. Not Prolog, though: Prolog has terms with free variables and unification. Those have no simple "database" interpretation.

The featured article only shows off a small (non-recursive, negation-free) fragment of a Datalog-like language, which itself is only a small (function-free) fragment of Prolog. You can't make broad claims about Prolog from this small set of examples.

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