Identifying the next unwatched episode in a series - sql

Question

I need to write a query in SQL that will identify the next unwatched episode in a series by a user. I have tables containing user history (userID, episodeID and show titleID) and Episode (episodeID(PK), episodeName and playCount), where the history table stores the episodeIDs of all watched episodes and the episode table contains all possible episodes.
I had hoped to do some sort of join between the two selecting by 'userID, titleID but i'm going in circles.
I've been learning SQL for about 5 hours in total so have no idea where to begin on this, any help gratefully received. I'm aware the code below is horrible and very clunky (please don't assume any prior knowledge!)
I've tried a left join, but this will return all data irrespective of user.
Create table Actor(
actorID integer primary key,
first_name text not null,
last_name text not null
);
create table AppearsIn(
actorID integer,
seriesID integer primary key
);
create table Series(
seriesID text,
titleID text,
episodeID text,
releaseDate date
);
create table Episode(
episodeID text primary key,
episodeName text,
playCount integer,
actorID integer
);
create table title(
titleID text primary key,
actorID integer,
titleName text,
genre text,
ageRating integer,
releaseDate date
);
create table history(
userID integer,
titleID text,
episodeID text
);
create table user (
userID integer not null,
fullName text not null,
Email text not null,
dateOfBirth date not null,
subscriptionEndDate date not null
);
insert into Actor (actorID, first_name, last_name)
values (76547, 'tom', 'cruise');
insert into Actor (actorID, first_name, last_name)
values (345, 'val', 'kilner');
insert into AppearsIn (actorID, seriesID)
values (345, 1);
insert into Series (seriesID, titleID, episodeID, releaseDate)
values ('WDS2', 'WalkingDead', 'WDS2E1', '1991-02-02');
insert into Series (seriesID, titleID, episodeID, releaseDate)
values ('WDS2', 'WalkingDead', 'WDS2E2', '1991-02-02');
insert into Series (seriesID, titleID, episodeID, releaseDate)
values ('WDS2', 'WalkingDead', 'WDS2E3', '1991-02-02');
insert into Series (seriesID, titleID, episodeID, releaseDate)
values ('WDS2', 'WalkingDead', 'WDS2E4', '1991-02-02');
insert into Series (seriesID, titleID, episodeID, releaseDate)
values ('WDS2', 'WalkingDead', 'WDS2E5', '1991-02-02');
insert into Series (seriesID, titleID, episodeID, releaseDate)
values ('WDS2', 'WalkingDead', 'WDS2E6', '1991-02-02');
insert into Series (seriesID, titleID, episodeID, releaseDate)
values ('WDS2', 'WalkingDead', 'WDS2E7', '1991-02-02');
insert into Series (seriesID, titleID, episodeID, releaseDate)
values ('WDS2', 'WalkingDead', 'WDS2E8', '1991-02-02');
insert into Episode (episodeID, episodeName, actorID, playCount)
values ('WDS2E1', 'A', 76547, 1);
insert into Episode (episodeID, episodeName, actorID, playCount)
values ('WDS2E2', 'B', 76547, 1);
insert into Episode (episodeID, episodeName, actorID, playCount)
values ('WDS2E3', 'C', 76547, 1);
insert into Episode (episodeID, episodeName, actorID, playCount)
values ('WDS2E4', 'D', 76547, 0);
insert into Episode (episodeID, episodeName, actorID, playCount)
values ('WDS2E5', 'E', 76547, 0);
insert into history (userID, titleID, episodeID)
values (8924, 'Walking Dead','WDS2E1');
insert into history (userID, titleID, episodeID)
values (8924, 'Walking Dead', 'WDS2E2');
insert into history (userID, titleID, episodeID)
values (8924, 'Walking Dead', 'WDS2E3');
insert into user (userID, fullName, Email, dateOfBirth, subscriptionEndDate)
values (8924, 'bill123', 'bill123#warmpost.com', '1970-02-12', '2019-06-05');
insert into title (titleID, actorID, titleName, genre, ageRating, releaseDate)
values ('WalkingDead', 123455, 'The Walking Dead', 'Drama', 15, '2015-01-01');
This is the query I tried:
SELECT * FROM Episode
LEFT JOIN history ON history.episodeID = Episode.EpisodeID
Where playcount <1
limit 1;

Answer 1

It sounds like you're just having issues with the join. Here's a page on how joins work. From that page:
(INNER) JOIN: Returns records that have matching values in both tables
LEFT (OUTER) JOIN: Return all records from the left table, and the matched records from the right table
RIGHT (OUTER) JOIN: Return all records from the right table, and the matched records from the left table
FULL (OUTER) JOIN: Return all records when there is a match in either left or right table
So, if you want only movies that user has seen, you could try replacing LEFT JOIN with RIGHT JOIN, or change the order in which you specify the tables.
I'm currently not able to test this myself, but hopefully this is what you're looking for.
SELECT * FROM Episode
RIGHT JOIN history ON history.episodeID = Episode.EpisodeID
WHERE playcount < 1
LIMIT 1;