SQLite : how to achieve ORM "eager loading" in plain SQL? - sql

Question

I have a SQLite database with Movies, Actors, and Tags.
There is a many-to-many relation between movies and actors, and movies and tags.
In my app, I want to list all movies with their corresponding actors and tags, for example:
Mr. & Mrs. Smith: Actors: Brad Pitt, Angelina Jolie, Tags: Action, Comedy, Crime
Passengers: Actors: Jennifer Lawrence, Chris Pratt, Tags: Adventure, Drama, Romance
And I'm wondering what are the correct SQL statements to achieve that.
The tables in my database are defined as follows :
CREATE TABLE "Movie"
(
id INTEGER PRIMARY KEY,
name VARCHAR
)
CREATE TABLE "Actor"
(
id INTEGER PRIMARY KEY,
name VARCHAR
)
CREATE TABLE "Tag"
(
id INTEGER PRIMARY KEY,
name VARCHAR
)
CREATE TABLE "Movie_Actor"
(
movie_id INTEGER,
actor_id INTEGER,
FOREIGN KEY(movie_id) REFERENCES "Movie" (id),
FOREIGN KEY(actor_id) REFERENCES "Actor" (id),
UNIQUE(movie_id, actor_id)
);
CREATE TABLE "Movie_Tag"
(
movie_id INTEGER,
tag_id INTEGER,
FOREIGN KEY(movie_id) REFERENCES "Movie" (id),
FOREIGN KEY(tag_id) REFERENCES "Tag" (id),
UNIQUE(movie_id, tag_id)
);
To get a single movie with it's actors and tags I use the following 3 queries (for example Movie.id = 1):
To get the movie row:
SELECT *
FROM Movie
WHERE id = 1
To get the actors:
SELECT *
FROM
(SELECT * FROM Actor) AS T1
JOIN
(SELECT * FROM Movie_Actor WHERE Movie_Actor.movie_id = 1) AS T2 ON T1.id = T2.actor_id
To get tags:
SELECT *
FROM
(SELECT * FROM Tag) AS T1
JOIN
(SELECT * FROM Movie_Tag WHERE Movie_Tag.movie_id = 1) AS T2 ON T1.id = T2.tag_id
My question is, how should I go about retrieving the tags and actors when I'm getting a list of movies such as SELECT * FROM Movie?
Many ORMs have an option to 'eager load' relations, and I'm wondering how can I do it in plain SQL?
Do I need to execute extra 2 queries on each row I get from SELECT * FROM Movie?
Thank You.

Answer 1

To get movie with id = 1 along with all of the actors associated with that movie you do the following:
SELECT * FROM Movie
LEFT JOIN Movie_Actor ON Movie_Actor.movie_id = Movie.id
LEFT JOIN Actor ON Actor.id = Movie_Actor.actor_id
WHERE id = 1
To also get all the tags, keep joining the associated tables Movie_Tag and Tag.
You might think that this would be terribly inefficient because a lot of information is going to be duplicated, for example the name of a movie is going to be fetched not just once, but NA * NT times, where NA is the number of fetched actors and NT is the number of fetched tags.
Actually, databases tend to be smart about that, (precisely because this is a very popular mechanism of retrieving data with as few as possible roundtrips to the database,) so within their communication protocols they contain special measures to avoid transmitting field values that are identical from row to row. So, the actual amount of data transmitted is very close to exactly the amount of data that would have been transmitted if you queried each table separately.
The benefit, of course, is that you suffer the penalty of a single round-trip to the database, instead of several round-trips, one for each table.