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How is Prolog ideal for when you'd use SQL? I can understand it if you're talking about querying a CSV file, but the advantage to SQL is that the database with

How is Prolog ideal for when you'd use SQL? I can understand it if you're talking about querying a CSV file, but the advantage to SQL is that the database with all your data natively supports SQL and it can usually run pretty fast. Btw: what would it look like to query a small sample CSV file. Like the below example if you wanted to find all cars that are made by Ford? Brand,Model,Year,Color/n Ford,F150,1999,Black/n Toyota,Yaris,2013,Yellow/n Ford,Mustang,1969,Blue/n Edit: sorry for not knowing how to format HN properly and show the newline on each of the rows.

hjek 7y ago
Hey, this is good practice! First we can load the csv library, then read the rows of the csv file, and assert those rows as facts using the `car` identifier. (Doesn't mean `car` like in lisp, obviously, but automobile.) ?- [library(csv)], csv_read_file('cars.csv',Rows, [functor(car)]), maplist(assert, Rows). If we want to find a Ford, then we can enter this query: ?- car('Ford', Model, Year, Colour). Any capitalised word that's not in quotes is a variable in Prolog, so they can take any value. This will give us the result: Model = 'F150', Year = 1999, Colour = 'Black' ; Press space, and we get another: Model = 'Mustang', Year = 1969, Colour = 'Blue'. And there are no more, so it returns to the REPL. (I should really have discarded the first line, because right now there's a car, whose brand is 'Brand', and so on. Well, well...) If we just wanted a list of all the results, we could use the findall/3 predicate. Something that's neat in Prolog is that you can define recursive relations. In SQL it's easy to define a parent/child relation, but what about ancestor/descendant? (Yes, you can use common table expressions, but the syntax is incredibly complicated.) Also, you don't need to create database schemas and Prolog, and unification feels seamless as opposed to explicitly declaring the JOINs. But, in my opinion, where Prolog really shines compared to SQL is that there just isn't that whole object-relational impedance mismatch[0]. [0]: https://en.wikipedia.org/wiki/Object-relational_impedance_mismatch https://en.wikipedia.org/wiki/Object-relational_impedance_mi... Edit: Ok, my turn quiz!! Say we have these bicycles: Brand, Colour, Year Mosquito, Blue, 2010 Raleigh, Red, 2005 Suzuki, Red, 1998 In SQL, how would you find the names of any two brands whose bicycles have the same colour? (Don't bother with the whole CSV parsing; just assume it's already a table.) In Prolog that would be: bicycle(One_brand, Colour, _), bicycle(Another_brand, Colour, _), One_brand \= Another_brand. Maybe I just have particularly hard time understanding SQL, but I'd have to surf StackOverflow for at very least half an hour to do that in SQL. I wouldn't know where to start.
- new4thaccount 7y ago
  Thanks for working through my example question. I may have to give Prolog another shot at some point!
- krackers 7y ago
  >how would you find the names of any two brands whose bicycles have the same colour? At first glance I thought it'd just be a group by + having followed by some sort of SELECT to get the first 2 in each group. But apparently doing that top-k in each group is a nightmare in SQL [1]. [1] https://stackoverflow.com/questions/8748986/get-records-with-highest-smallest-whatever-per-group https://stackoverflow.com/questions/8748986/get-records-with...
- yawaramin 7y ago
  Nice quiz :-) My attempt would be: select b1.Brand as brand1, b2.Brand as brand2 from bicycles as b1 inner join bicycles as b2 on (b1.Colour = b2.Colour) where b1.Brand <> b2.Brand
  - hjek 7y ago
    Cool! That's actually really close to the Prolog version.
    - yawaramin 7y ago
      True that. Only major difference is that I had to explicitly specify how to relate the two brands together in SQL whereas in Prolog it's implicit by using the same variable in two different patterns. I find the idea of a simple Prolog, like a Datalog, very intriguing as a potential simple query engine for apps in mainstream languages–not having to worry about the maps, filters, reduces, flatmaps, and all the other operations we have to do to coax data into the right shapes in line-of-business apps. Unfortunately I just haven't been able to figure out how to do a left outer join (or if that's even the right approach in Prolog :-)
      - 7y ago

hjek 7y ago

Hey, this is good practice! First we can load the csv library, then read the rows of the csv file, and assert those rows as facts using the `car` identifier. (Doesn't mean `car` like in lisp, obviously, but automobile.) ?- [library(csv)], csv_read_file('cars.csv',Rows, [functor(car)]), maplist(assert, Rows). If we want to find a Ford, then we can enter this query: ?- car('Ford', Model, Year, Colour). Any capitalised word that's not in quotes is a variable in Prolog, so they can take any value. This will give us the result: Model = 'F150', Year = 1999, Colour = 'Black' ; Press space, and we get another: Model = 'Mustang', Year = 1969, Colour = 'Blue'. And there are no more, so it returns to the REPL. (I should really have discarded the first line, because right now there's a car, whose brand is 'Brand', and so on. Well, well...) If we just wanted a list of all the results, we could use the findall/3 predicate. Something that's neat in Prolog is that you can define recursive relations. In SQL it's easy to define a parent/child relation, but what about ancestor/descendant? (Yes, you can use common table expressions, but the syntax is incredibly complicated.) Also, you don't need to create database schemas and Prolog, and unification feels seamless as opposed to explicitly declaring the JOINs. But, in my opinion, where Prolog really shines compared to SQL is that there just isn't that whole object-relational impedance mismatch[0]. [0]: https://en.wikipedia.org/wiki/Object-relational_impedance_mismatch https://en.wikipedia.org/wiki/Object-relational_impedance_mi... Edit: Ok, my turn quiz!! Say we have these bicycles: Brand, Colour, Year Mosquito, Blue, 2010 Raleigh, Red, 2005 Suzuki, Red, 1998 In SQL, how would you find the names of any two brands whose bicycles have the same colour? (Don't bother with the whole CSV parsing; just assume it's already a table.) In Prolog that would be: bicycle(One_brand, Colour, _), bicycle(Another_brand, Colour, _), One_brand \= Another_brand. Maybe I just have particularly hard time understanding SQL, but I'd have to surf StackOverflow for at very least half an hour to do that in SQL. I wouldn't know where to start.

new4thaccount 7y ago
Thanks for working through my example question. I may have to give Prolog another shot at some point!
krackers 7y ago
>how would you find the names of any two brands whose bicycles have the same colour? At first glance I thought it'd just be a group by + having followed by some sort of SELECT to get the first 2 in each group. But apparently doing that top-k in each group is a nightmare in SQL [1]. [1] https://stackoverflow.com/questions/8748986/get-records-with-highest-smallest-whatever-per-group https://stackoverflow.com/questions/8748986/get-records-with...
yawaramin 7y ago
Nice quiz :-) My attempt would be: select b1.Brand as brand1, b2.Brand as brand2 from bicycles as b1 inner join bicycles as b2 on (b1.Colour = b2.Colour) where b1.Brand <> b2.Brand
- hjek 7y ago
  Cool! That's actually really close to the Prolog version.
  - yawaramin 7y ago
    True that. Only major difference is that I had to explicitly specify how to relate the two brands together in SQL whereas in Prolog it's implicit by using the same variable in two different patterns. I find the idea of a simple Prolog, like a Datalog, very intriguing as a potential simple query engine for apps in mainstream languages–not having to worry about the maps, filters, reduces, flatmaps, and all the other operations we have to do to coax data into the right shapes in line-of-business apps. Unfortunately I just haven't been able to figure out how to do a left outer join (or if that's even the right approach in Prolog :-)
    - 7y ago