Accessing Data

Overview

Logica rules define how facts are inferred from other facts. But where do the initial facts come from? In most real programs, data comes from database tables or files. This page explains how to bring external data into your Logica program.

Database Tables

When Logica encounters a predicate that is not defined anywhere in your program, it treats it as a database table and translates it directly into a FROM clause in the generated SQL.

For example, suppose your database has metropolies schema (or dataset as BigQuery calls it) and a table within it with the full name metropolies.City. The table has columns name and country. You can use it in a rule just like any other predicate:

CapitalCity(name) :- metropolies.City(name:, is_capital: true);

Logica will query the City table from your database, filtering rows where is_capital is true, and return the name column.

TIP

Any unknown predicate is passed through to SQL as a table reference. No special declaration is needed.

You can combine database tables with conjunctions and disjunctions just as you would with any other Logica predicate:

# Cities that are capitals or have population over 1 million.
BigOrCapital(name) :-
  metropolies.City(name:, is_capital: true) |
  metropolies.City(name:, population:), population > 1000000;

Arbitrary Table Expressions

Sometimes you need to call a table-valued function — a function that returns a table rather than a single value. In SQL these appear in the FROM clause. Logica supports this with the backtick syntax:

`(<sql expression>)`

The expression inside the backticks is placed directly into the SQL FROM clause. For example, DuckDB's read_text function reads a file and returns a table:

FileLines(content) :- `(read_text("data.txt"))`(content:);

TIP

The backtick syntax is a purely syntactic transformation: `(expr) is replaced with expr in the FROM clause. It works the same way on all supported engines. In fact Logica complier doesn't know if you are using a table of a table-valued function, it simply places your expression into the SQL.