Hi,
I've started learning Prolog and wondering about theoritical difference from SQL language. For example they are both declarative paradigma languages, both support fact-driven knowledge database, both support question-styled data-retrieving, both support functional dependencies. So does anybody have an idea?
Tnx in advance.
I think the main difference is that Prolog is a query language used for matching complicated patterns against a database of simple facts. SQL on the other hand is limited to relational databases.
There are many differences which I think become clear when you start using them. Remember because of changes in terminology, somethings which are called the same thing in the past mean very different things now.
Very broad overview of difference.
SQL statements work against a relational database and query (ask for) data from that database, changes to that data and the results are exactly expressed in the language, whereas in Prolog you define facts and a logic engine generates new facts based off of the existing facts. New data (facts) are created via evaluation.
They both use something called queries (but they work totally differently) and they both have data (but use it differently.)
The use cases for SQL and Prolog are also totally different. It would never make sense to store an address list in Prolog whereas that is exactly what SQL was designed to do.
Simply put SQL is used to access a data store and Prolog is an expression evaluator.
Just a few thoughts:
Most of the incorrect answers here are a reflection of the fact that most people do not know what SQL is (its an implementation of Relational Calculus) or what that means (that it's a form of Predicate Logic). The following statements are true of both Prolog and SQL:
Generally, people are not aware of these equivalences between them:
So what are their differences? Although they operate across the same conceptual domains, their focuses are in completely different directions. In Prolog terms, SQL is primarily a Fact and Relation(set) engine, whereas Prolog is primarily a Rules and Inferencing engine. Each can do the other, to a limited extent, but it becomes increasingly difficult with even small increases in complexity. For instance, you can do inferencing in SQL, but it is almost entirely manual in nature and not at all like the automatic forward-inferencing of Prolog. And yes, you can store data(facts) in Prolog, but it is not at all designed for the "storage, retrieval, projection and reduction of Trillions of rows with thousands of simultaneous users" that SQL is.
Plus, SQL is primarily a Server-language paradigm, whereas Prolog is primarily a Client-language paradigm.
This will get you started: Programming in Prolog
Very early on, you can read this:
Prolog, as a programming language, is a little unusual. It can be understood as a standard procedural language with two unusual properties. It is a procedural language like Pascal or Algol. One programs in a procedural language by writing procedures that carry out particular operations. One specifies what a procedure is to do by using primitive statements and by invoking other procedures. Prolog procedures have only local variables and all the information that a procedure can use or produce must be passed through its arguments.
C can be viewed as a procedural language by thinking of using it without functions; i.e., all functions return void, and information is passed to and from functions through their arguments only.
In Prolog, procedures are called predicates. The two unusual aspects of Prolog are:
1. Prolog has assign-once variables, and 2. Prolog is nondeterministic.
Enjoy.
You are correct: Prolog and SQL are theoretically related (you asked specifically about theoretical differences).
I want to complement RBarryYoung's answer, giving you some hints to understand the connection, so that you have a starting point to study and understand the technicalities.
Prolog and SQL share a core: every query expressible in a subset of Prolog can be expressed in a subset of SQL and viceversa, i.e. these subsets are logically equivalent.
To understand how this can be true, you need to examine on what theoretical underpinnings both Prolog and SQL are based:
Of course something out of these subsets needs more translation effort.
Nonetheless, I think the claim that equivalence in expressive power of the two subsets is more than an to appeal to Turing equivalence4 when considering Prolog-to-SQL translation.
Notes:
1) Unfortunately SQL can be used in contrast to RDBMS theoretical foundations (relational algebra-calculi); for example, SQL tables are not necessarily relations - as per RA - i.e. they can be without a (primary) key, so duplicate rows are permitted. Such tables are not sets but multisets (aka bags) of tuples. In such context, all theoretical results for RA, where relations are sets, are not necessarily valid.
2) For a translation from SQL to TRC, see A note on the translation of SQL to tuple calculus, also here (postscript paper).
3) For more on the differences between Datalog and Prolog see What You Always Wanted to Know About Datalog (And Never Dared to Ask) (pdf paper - links directly to page 6, heading H. Datalog and Prolog).
4) For the record: RA (and so their equivalents safe TRC and safe Datalog w/o recursion) is not Turing complete on purpose, to avoid never ending queries.
Historical note: Prolog and Codd's Relational Algebra were conceived around the same time (end of '60s early '70s) in different contexts - Colmerauer conceived Prolog for natural language processing and Codd conceived RA as a theoretical foundation of Relational DBMS. So, any theoretical connection between Prolog-Datalog-RA-SQL was necessarily established a posteriori and is implicit in the fact that they are all based on first-order predicate calculus (aka first order logic).