DBQUERY                                                     Chris Mellish
                                                            September 1983

A DATABASE QUERY LANGUAGE
-------------------------

The Prolog library package DBQUERY provides an alternative language for
querying a Prolog database. This language is a suitable target for natural
language understanding programs dealing with database queries.

Expressions in the query language are POPLOG lists representing propositions
in a manner similar to Predicate Logic. When a query is presented for
evaluation, the system attempts to verify that the complex proposition it
represents is true. As a side effect of verifying the truth of the main
proposition, it may have to verify the truth of various subsidiary
propositions. Some special propositions are always true and cause things
(ANSWERS) to be printed out when they are encountered.

Here are the kinds of expressions that can appear in this language. In the
language, variables are represented by words consisting of the letter "X"
followed by a number (NB The POP-11 notation for lists is used here):

    [all VARIABLE CONDITION GOAL]

    To check that this is true involves finding all the values of the VARIABLE
which make the CONDITION true and then checking that for each of these values
the GOAL is also true.

    [exists VARIABLE CONDITION GOAL]

    To check that this is true involves checking that there is a value of the
VARIABLE that makes both the CONDITION and the GOAL true.

    [the VARIABLE CONDITION GOAL]

    The checking for this is as in the last case, except that, if the check
does not succeed it is considered that there is an error in the question.

    [and CONDITION1 CONDITION2]

    To check this, check that both CONDITION1 and CONDITION2 are true.

    [not CONDITION]

    This checks out as true if the CONDITION checks out as false, and vice
versa.

    [printout VARIABLE]

    This always checks out as true, but whenever it is encountered the current
value of the VARIABLE is printed out.

    [PREDICATE ...ARGUMENTS...]

    This represents a proposition that can be verified by directly looking at
the database (and possibly using Prolog inference rules). PREDICATE is a word
specifying the relation involved and ARGUMENTS specify the objects involved in
that relation. What predicates you can sensibly use and what arguments you can
sensibly provide depend on what is in your database.

Here are some examples of queries in this language, together with natural
language questions that they might correspond to:

    [all X1
        [man X1]
        [printout X1]
    ]
    "what men are there?"

   [all X1
       [and [person X1]
           [exists X2 [monarch X2] [bore X1 X2]
               ]
           ]
       [printout X1]
       ]
    "who bore a monarch?"

    [all X1
        [and [person X1] [exists X2 [bore X1 X2]]]
        [printout X1]]
    "which people have children?"

You may notice a similarity between these forms and certain constructions in
POP-11 and the Predicate Calculus.

Using the System
----------------

The library file does not provide any special facilities for CONSTRUCTING
database queries - that is up to other programs (eg natural language
understanders) to provide. What it provides is the ability to answer a query
that has already been constructed. The POP-11 procedure QUERY takes a list as
argument and answers the query. A Prolog predicate, 'query' with 1 argument,
provides the same capability.

Shortcomings
------------

The way in which the system prints out answers to complex questions leaves a
lot to be desired. Its error messages are also unhelpful - ideally some
natural language generation is called for here (see "Cooperative Responses
from a Portable Natural Language Database Query System" by J. Kaplan in
"Computational Models of Discourse, edited by Brady and Berwick, MIT Press,
1983).

References
----------

There are great similarities between this query language and that used by
Woods in the LUNAR system. The following references cover some work on the use
of similar languages for natural language querying of databases:

Colmerauer, A., "An interesting subset of Natural Language", in Clark, K.L.
and Taernlund, S. A., (Eds) LOGIC PROGRAMMING, Academic Press, 1982.

Woods, W. A., "Semantics for Question Answering System", especially Chapter 9.

Woods, W. A. paper in Zampolli, A. "Linguistic Structures Processing", North
Holland.

Woods, W. A. paper in "Advances in Computers", Vol 17, Ed. M. C. Yovits.

Warren, D. and Pereira, F., "An Efficient Easily Adaptable System for
Interpreting Natural Language Queries", AJCL Vol 8, Number 3-4, 1982.

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