FAQ: What Are Database Relationships? - One-to-One Relationship

Frequently Asked Questions SQL FAQ
1

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This community-built FAQ covers the “One-to-One Relationship” exercise from the lesson “What Are Database Relationships?”.

Paths and Courses
This exercise can be found in the following Codecademy content:

Design Databases With PostgreSQL

FAQs on the exercise One-to-One Relationship

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2

I’m getting an error in the last step of this exercise and I can’t figure out why. The previous step had me copy and paste all the “INSERT” statements into this code and it originally included one more insert statement that I have since removed that was meant to purposely throw an error. Any help is appreciated.

‘isbn’ is the primary key for the ‘book’ table, and ‘book_isbn’ is the foreign key that relates to it from the ‘book_details’ table.

  id integer PRIMARY KEY,
  book_isbn varchar(50) REFERENCES book(isbn) UNIQUE,
  rating decimal,
  language varchar(10),
  keywords text[],
  date_published date
);

SELECT
  constraint_name, table_name, column_name
FROM
  information_schema.key_column_usage
WHERE
  table_name = 'book_details';

SELECT
  constraint_name, table_name, column_name
FROM
  information_schema.key_column_usage
WHERE
  table_name = 'book';

INSERT INTO book VALUES (
  'Learn PostgreSQL',
  '123457890',
  100,
  2.99,
  'Great course',
  'Codecademy'
);

INSERT INTO book_details VALUES (
  1,
  '123457890',
  3.95,
  'English',
  '{sql, postgresql, database}',
  '2020-05-20'
);

SELECT
  book.title,
  book.price,
  book_details.language,
  book_details.rating
FROM
  book, book_details
WHERE
  book.isbn = book_details.book_isbn;
3

I just now realized there is a “Show Solution” button. lol
It appears that the problem came from me deleting the aforementioned code, and also from me adding the code to check the primary key of the ‘book’ table.

1 Like
4

Solution for last step using the method not given in the solution:

select book.title, book.price, book_details.language, book_details.rating
from book, book_details
where book.isbn = book_details.book_isbn

5

Why does the ‘license’ table have both columns ‘id’ (which is the primary key) and ‘license id’ ? Can’t the license id serve as the id in this table as it is unique and to avoid a redundant column?image

1 Like
6

Yes, I have the same question, anyone has an idea ?

1 Like
7

I have found that running the code below in script.sql works which omits the JOIN query :

SELECT
book.title,
book.price,
book_details.language,
book_details.rating

FROM book, book_details;

I wanted to ask why a JOIN query is being used in the solution when we have already ‘joined’ both tables together in effect when creating the tables and linking via REFERENCE & UNIQUE?