What Does it Look Like?

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(Disclaimer: what you are about to see is a glimpse of application code that uses quince, largely without explanation. A more detailed guided tour can be found here, and more detail still here.)

Consider the following task: print the titles of all the movies that are now playing on some screen in some cinema within a given radius of a given point.

If we use the following C++ data types:

struct point {
    float x;
    float y;

struct movie {
    serial id;                  // primary key with automatically assigned values
    std::string title;

struct cinema {
    serial id;                  // primary key with automatically assigned values
    point location;

struct screen {
    serial id;                  // primary key with automatically assigned values
    serial cinema_id;           // foreign key to cinemas table
    serial current_movie_id;    // foreign key to movies table

and we assume that the following have been defined elsewhere:

extern serial_table<movie> movies;
extern serial_table<cinema> cinemas;
extern serial_table<screen> screens;

extern point my_place;
extern float max_radius;

Then one way to construct a suitable query is:

const query<std::string> titles_playing_nearby =
           (cinemas->location.x - my_place.x) * (cinemas->location.x - my_place.x)
         + (cinemas->location.y - my_place.y) * (cinemas->location.y - my_place.y)
         <= max_radius * max_radius
    .inner_join(screens, screens->cinema_id == cinemas->id)
    .inner_join(movies, movies->id == screens->current_movie_id)

Now, to generate the required report, we use a C++11 for-loop. This causes the SQL query to be generated and executed, and the loop body to be run as each row of output arrives:

for (const std::string &title: titles_playing_nearby)
    std::cout << title << std::endl;