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[fit] SQL Joins


Tracking information that isn't a single value.

In relationship to other data.


[fit] Let's go to the movies


pgcli SuncoastMovies

Primary Key

  • Relating information between multiple tables.
  • Need a way to uniquely identify a row of data in a table.
  • Known as the table's PRIMARY KEY.
  • Uniquely identifies the row and also cannot repeat.

Primary Key

  • You might have an existing column that you feel uniquely identify the row.
  • You might think that the movie's title would uniquely identify the movie. However, we know that sometimes a movie's title changes during the production, or even uses the name of a movie that has existed in the past.
  • So if we used the title to uniquely identify it we would run into many issues.

SERIAL Data Type

  • Databases provide their own way of supplying a unique value for each row in the database.
  • In Postgres We call this SERIAL column.
  • A SERIAL data type will begin at 1 and increase for each new row.
  • Values are never reused or repeated.

Defining a Primary Key for our Movies

[.column]

  • Use Id column name. This is a common pattern.
  • Define the data type as: SERIAL.
  • Denote that this column is part of the PRIMARY KEY

[.column]

Data Type
|
| Mark this column as part of the PRIMARY KEY
| |
| |
v v---------v
"Id" SERIAL PRIMARY KEY,

CREATE TABLE "Movies" (
"Id" SERIAL PRIMARY KEY,
"Title" TEXT NOT NULL,
"PrimaryDirector" TEXT,
"YearReleased" INT,
"Genre" TEXT
);

Add some movies

INSERT INTO "Movies" ("Title", "PrimaryDirector", "YearReleased", "Genre")
VALUES ('The Lost World', 'Steven Spielberg', 1997, 'sci-fi');
INSERT INTO "Movies" ("Title", "PrimaryDirector", "YearReleased", "Genre")
VALUES ('Pirates of the Caribbean: The Curse of the Black Pearl', 'Gore Verbinski', 2003, 'fantasy');
INSERT INTO "Movies" ("Title", "PrimaryDirector", "YearReleased", "Genre")
VALUES ('Harry Potter and Goblet of Fire', 'Mike Newell', 2005, 'fantasy');
INSERT INTO "Movies" ("Title", "PrimaryDirector", "YearReleased", "Genre")
VALUES ('The Hobbit: An Unexpected Journey', 'Peter Jackson', 2012, 'fantasy');

Foreign Keys

  • In order to keep track of the rating for any given movie we will add a single table, named Ratings that will store the name of the rating.
  • Since we also want to uniquely identify the ratings, we'll ensure this table also has a serial primary key.

Ratings

CREATE TABLE "Ratings" (
"Id" SERIAL PRIMARY KEY,
"Description" TEXT
);

Let's insert some ratings:

INSERT INTO "Ratings" ("Description") VALUES ('G');
INSERT INTO "Ratings" ("Description") VALUES ('PG');
INSERT INTO "Ratings" ("Description") VALUES ('PG-13');
INSERT INTO "Ratings" ("Description") VALUES ('R');

Actors

  • Let's also add a table to keep information about our actors.
  • For this table we want to know the full name of the actor and their birthday.
  • We'll also create an Id that is a PRIMARY KEY and is SERIAL
CREATE TABLE "Actors" (
"Id" SERIAL PRIMARY KEY,
"FullName" TEXT NOT NULL,
"Birthday" DATE
);

Actors

INSERT INTO "Actors" ("FullName", "Birthday")
VALUES ('Orlando Bloom', '1977-01-13');
INSERT INTO "Actors" ("FullName", "Birthday")
VALUES ('Warwick Davis', '1970-02-03');
INSERT INTO "Actors" ("FullName", "Birthday")
VALUES ('Martin Freeman', '1971-09-08');

Relationships

+----------------------------+ +-----------------------+
| Movies | | Ratings |
| | | |
| Id SERIAL | | |
| Title TEXT | | Id SERIAL |
| PrimaryDirector TEXT | | Description TEXT |
| YearReleased INT | | |
| Genre TEXT | +-----------------------+
+----------------------------+

Relationship

Let's add a new column to our Movies to indicate WHICH rating is associated to each row representing a movie.


ALTER TABLE "Movies" ADD COLUMN "RatingId" INTEGER NULL REFERENCES "Ratings" ("Id");
  • RatingId is an integer since it matches the SERIAL which we are going to relate to.
  • NULL indicates that we are allowed to have no value.
  • Next we indicate that this is a foreign key (we are relating this table) to the Ratings table.
  • We also specify the column in the other table, in this case Id in Ratings, we mean to match.

+----------------------------+ +-----------------------+
| Movies | | Ratings |
| | | |
| Id SERIAL | +---------> Id SERIAL |
| Title TEXT | | one | Description TEXT |
| PrimaryDirector TEXT | | | |
| YearReleased INT | | | |
| Genre TEXT | many | +-----------------------+
| RatingId INT <--------+
| |
+----------------------------+

Now we can specify the RatingId associated to each movie when we insert the movie.

UPDATE "Movies" SET "RatingId" = 2 WHERE "Id" IN (10);
UPDATE "Movies" SET "RatingId" = 3 WHERE "Id" IN (1, 2, 3, 4, 5, 6, 7, 8, 9 );
UPDATE "Movies" SET "RatingId" = 4 WHERE "Id" IN (11, 12, 13 );

Joining tables

So now that we have these two tables, how do we join them together so that we can retrieve information about movies and their ratings or get information about a rating and the associated movies.


[fit] Query movies and also get their rating

SELECT *
FROM "Movies"
JOIN "Ratings" ON "Movies"."RatingId" = "Ratings"."Id";

[fit] Query for all movies that are "R", adding a WHERE clause

SELECT *
FROM "Movies"
JOIN "Ratings" ON "Movies"."RatingId" = "Ratings"."Id"
WHERE "Ratings"."Description" = 'R';

  • This query will give us movies and their ratings.
  • But only for movies that have a RatingId that matches an Id from the ratings table.
  • That is, any movie with a null value for RatingId (or a value that doesn't match an id) will not be in the results.

fit


[fit] Query all the movies and include ratings when possible

SELECT *
FROM "Movies"
LEFT JOIN "Ratings" ON "Movies"."RatingId" = "Ratings"."Id";

Many to Many

+--------------------------------+ +---------------------------+
| Movies | | Ratings |
| | | |
| Id SERIAL | | Id SERIAL |
| Title TEXT |many one| Description TEXT |
| PrimaryDirector TEXT +-----------+ |
| YearReleased INT | +---------------------------+
| Genre TEXT |
| RatingId INT |
| |
+------------+-------------------+
|
| many
|
|
| +-------------------------+
| | ACTORS |
| | |
| many | Id SERIAL |
+-------------------+ FullName TEXT |
| Birthday DATE |
| |
+-------------------------+

  • In the case of a many-to-many relationship we cannot place the foreign keys on either of the tables.
  • In this case we need a third table, commonly referred to as a join table to store the relationships.
  • In this table, we will place two foreign keys, one to the left (movies) and the other to the right (actors).
  • We attempt to name this table based on the relationship between the two tables.

CREATE TABLE "Roles" (
"Id" SERIAL PRIMARY KEY,
"MovieId" INTEGER REFERENCES "Movies" ("Id"),
"ActorId" INTEGER REFERENCES "Actors" ("Id")
);

+--------------------------------+ +---------------------------+
| Movies | | Ratings |
| | | |
| Id SERIAL | | Id SERIAL |
| Title TEXT | many one | Description TEXT |
| PrimaryDirector TEXT <-----------> |
| YearReleased INT | +---------------------------+
| Genre TEXT |
| RatingId INT |
| |
+-------------^------------------+
| one
|
|
|
|
| many
+-------v---------------+ +-------------------------+
| Roles | | Actors |
| | many one | |
| Id SERIAL <---------------> Id SERIAL |
| MovieId INT | | FullName TEXT |
| ActorId INT | | Birthday DATE |
| | | |
+-----------------------+ +-------------------------+

[fit] Insert some Roles


Query for the casts and actors

SELECT "Movies"."Title", "Actors"."FullName"
FROM "Movies"
JOIN "Roles" ON "Roles"."MovieId" = "Movies"."Id"
JOIN "Actors" ON "Actors"."Id" = "Roles"."ActorId";

[fit] Adding information to the join table.

  • What if we wanted to capture the name of the character the actor played?
  • It can't go on the Movies table since it isn't distinct to a movie. It can't go on the Actors table since it isn't unique to that either.
  • The correct place here is to place that column on the Roles table.

Updating the Roles table

Let's call this new column CharacterName and add it to the Roles table.

ALTER TABLE "Roles" ADD COLUMN "CharacterName" TEXT NULL;

Now that we have done that, we can add in a few character names. In order to know what rows to update, let's add the Roles.Id to our query above.

SELECT "Roles"."Id", "Movies"."Title", "Actors"."FullName", "Roles"."CharacterName"
FROM "Movies"
JOIN "Roles" ON "Roles"."MovieId" = "Movies"."Id"
JOIN "Actors" ON "Actors"."Id" = "Roles"."ActorId";

Now let's update the roles for all of our actors

-- Orlando Bloom played Will Turner in Pirates (ID 1)
UPDATE "Roles" SET "CharacterName" = 'Will Turner' WHERE "Id" IN (1);
-- Orlando Bloom played Legolas in the Lord of the Rings movies
UPDATE "Roles" SET "CharacterName" = 'Legolas' WHERE "Id" IN (2,3,4);

[fit] Rerun the query

SELECT "Roles"."Id", "Movies"."Title", "Actors"."FullName", "Roles"."CharacterName"
FROM "Movies"
JOIN "Roles" ON "Roles"."MovieId" = "Movies"."Id"
JOIN "Actors" ON "Actors"."Id" = "Roles"."ActorId";

If we started with a more detailed ERD we could have avoided the alter table statements for adding our relationships

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