Cities Database

Consider the following two examples:

CITY
NAME	STATE	POPULATION
Houston	Texas	3,000,00
Dallas	Texas	2,000.000
Huntsville	Texas	150,000
Austin	Texas	750,000
Corsicana	Texas	60,000
Shreveport	Luisiana	90,000
Bastrop	Texas	6,000
San Antonio	Texas	1,500,000

DISTANCE
CITY1	CITY2	MILES
Huston	Bastrop	130
Huston	Huntsville	60
Huntsville	Dallas	100
Austin	Waco	110
Waco	Dallas	100
Dallas	Shreveport	200
Austin	Bastrop	30
Austin	San Antonio	80
San Antonio	Huston	190

   Database: Facts and Schema

---

Facts

• Facts Corresponds to tuples in the database: For example,

city('Houston', 'Texas', 3000000).
distance('Austin', 'Waco', 110).
assembly(wheel, spoke, 36).

• Variables start with capital letters.
• Constants that begin with a capital letter are enclosed in single quotes.
• city('Houston', 'Texas', 30000) corresponds to the row
  ('Houston', 'Texas', 30000) in the city table.

database( { city(Name:string, State:string, Population:integer),
            distance(City1:string, City2:string, Distance:integer)
          } ).

• Each predicate (relation) is named.
• Each column is (optionally) named.
• Each column is assigned a data type.
• Datatypes are: integer, real, string, any.
• Columns of type any can contain complex terms (functors).

   Rules and Queries

---

• New (or derived) predicates are defined by rules.

• Example: derive the city in Texas with more than 400,000 people

  lt_city(C, Pop) <-  city(C, `Texas`, Pop), Pop > 400000. 
  

• Rules have a purely declarative role, similar to virtual views in relational databases

• Queries have an imperative role: when the query is executed then the results tha satisfy the query are returned

• Example: List all cities in Texas with where the population exceeds 400,000.

  query lt_city(C, Pop)
  

• This query will return:

     lt_city(`Huston`,   3000000)
     lt_city(`Dallas`,   2000000)
     lt_city(`Austin`,    750000)
     lt_city(`San Antonio`, 1500000)
  

• But queries without variables (aka closed queries) return a yes/no answer (no in this case since Austin does not have 2,000,000 inhabitants)

 query lt_city(`Austin`, 2000000) 

   Queries and Query Forms

---

• A query form, or export, is a generic query that specifies for the compiler which of the arguments will be given and which are expected as output when a query of that type is issued.
• The given bindings in an export allow the compiler to optimize the access to the data.
• Exports can be specified for base predicates as well as derived predicates that are defined via the programs' rules.

Then, the user can compile this export, and then run the following query:

The answers are then returned (but if no export match this bound/free pattern an error message is returned)

Example of Query Forms

• export lt_city($X, Y).
  X is given and Y is expected.
• export lt_city(X, $Y).
  X is expected and Y is given.
• export lt_city($X, $Y).
  Both X and Y are given and the response is True/False.

   Constructs

---

• Numbers---integer or real.
• Constants.

  A constant is a string of symbols beginning with a lowercase letter. Constants are names of objects such as chain_stay, joe. If we want to use constants starting with an uppercase letter, then we must enclose them in quotes, e.g., 'Houston'.

• Variables.

  A variable is a string of symbols beginning with an uppercase letter. Examples are Subpart, Price, X123. Variables denote unspecified values that are assigned during the execution of an program.

• Numbers, constants, and variables are included in the syntactic subclass called terms.