SQL database design for tags. How to handle missing relation between two tables? - postgresql

Question

I have three tables in a PostgreSQL database (one for storing articles, one for all tags, and one for the relation between the two):
table: article
columns: article_id, title, content
table: tag
columns: tag_id, name
table: article2tag
columns: article_id, tag_id
For now I only have two tags in the tag table:
tag (table)
-----------
tag_id name
1 apple
2 orange
I have an article (the one with ID 1) tagged with the two tags: apple and orange:
article2tag (table)
--------------------
article_id tag_id
1 1
1 2
But let's say one of the tags in the tag table will be removed, for example the tag named apple, now tag_id in article2tag will point to an inexistent tag.
What is the proper way (if there is one) to handle this situation?

Answer 1

That can be handled with foreign key constraints.
ALTER TABLE article2tag
ADD FOREIGN KEY (article_id)
REFERENCES article
(article_id);
ALTER TABLE article2tag
ADD FOREIGN KEY (tag_id)
REFERENCES tag
(tag_id);
That way, if you try to delete a tag that is still used in an article you'll get an error and cannot delete the tag.
You can also specify that, if a tag is deleted it will be removed from all articles with ON DELETE CASCADE.
ALTER TABLE article2tag
ADD FOREIGN KEY (tag_id)
REFERENCES tag
(tag_id)
ON DELETE CASCADE;
You can also use that for the foreign key to article.
You probably don't know about primary keys too, so you might encounter an error when trying to create the foreign key constraints about the referenced column not being unique. In that case add primary key constraints to article and tag.
ALTER TABLE article
ADD PRIMARY KEY (article_id);
ALTER TABLE tag
ADD PRIMARY KEY (tag_id);
You should also define a primary key for articel2tag analogously.
ALTER TABLE article2tag
ADD PRIMARY KEY (article_id,
tag_id);