Yashu Mittal

Posted on Feb 10, 2020

Best way to store array type data inside database?

#database #help #question #sql

What is the best way to store string of array inside a database column?

I have a tags columns, where I want to store multiple tags.

Now I can think of two possible way to this but not sure which is really effective for large-size data.

1st

Directly store the data in an array of a row.

[ { id: 1, name: 'action' }, { id: 2, name: 'classic' }, { id: 3, 'science fiction' } ]

If I want to add the data I push at the end of the array and for remove I can search it by ID.

2st

Create a separate tags table.

| id | name | post_id | tag_id |
| --- | --- | --- | 2 |
| 1 | 'action' | 342342 | 34 |
| 2 | 'classic' | 32424453 | 432 |
| 3 | 'science fiction' | 32424453 | 34 |

Based on the post_id in the tags table I can get all the tags for that specific post.

NOTE: These are not the actual tags but the reference where; which post is having which tag.

I would like to hear your feedback on these two ways, if you have another way to deal with array data type, please let me know in the comment section below.

Top comments (21)

Florian Rand • Feb 10 '20 • Edited

CREATE TABLE post (
`post_id` INT NOT NULL PRIMARY KEY,
`title` VARCHAR(50)
);

CREATE TABLE tags (
`tag_id` INT NOT NULL PRIMARY KEY
`tag_name` VARCHAR(20),
);

CREATE TABLE post_tags (
`post_id` INT NOT NULL,
`tag_id` INT NOT NULL,
PRIMARY KEY(`person_id`, `tag_id`)
);

The post_tags table contains the foreign keys of tags and posts.
Something like this (:

post_id	tag_id
1	1
1	5
1	3
2	2
2	1
2	3

To retrieve a post and all its tags, you need to JOIN the tables in your queries:

SELECT p.*, t.*
FROM post p
INNER JOIN post_tags pt
ON pt.post_id = p.post_id
INNER JOIN tags t
ON t.tag_id = pt.tag_id

This answer is adapted from How to store arrays in MySQL from StackOverflow.

Hassan Farid • Feb 10 '20

This is the best solution on the topic. +1

Yashu Mittal • Feb 10 '20

Thank you for sharing the code sample.

But, I want to share multiple tags for single post.

Florian Rand • Feb 10 '20

To retrieve a post and all its tags, you need to JOIN the tables in your queries

Yashu Mittal • Feb 12 '20 • Edited

On using the INNER JOIN clause it doesn't get the data if specific post have multiple tag_id

SELECT
    *
FROM
    post
INNER JOIN tag
ON post.post_id = tag.post_id;

it only return the data for 3rd row according to the above image.

Florian Rand • Feb 12 '20

INNER JOIN tag

Can you see the mistake here?

You need to do the JOIN with the post_tags table (with both references, posts and tags) to work.

Yashu Mittal • Feb 12 '20

Yeah I know that,

I tried with the both with and without post_tag table, it was return 0 rows.

rhymes • Feb 10 '20

Depends on what tags mean in your app, as in both examples have ids I guess they are part of your domain model. You can have a table which lists all available tags and another one with foreign keys to both posts and tags to list which posts have which tags.

Yashu Mittal • Feb 10 '20 • Edited

As I have mentioned in the article. A post can have multiple tags.

rhymes • Feb 10 '20

So yeah, having a many to many table is one solution. But you can also store them as an array or as JSONB in PostgreSQL. It depends if tags themselves are an entity in your app you want to attach metadata to or are just, well, an array of names you might want to query on (for example to give you the list of posts that have a specific tag)

Yashu Mittal • Feb 10 '20

Yes, tags itself have a separate entity in the app, to explain it in simpler words, this is how the data output I am expecting.

{
  postId: 3453,
  title: "sdf",
  slug: "sfs",
  tags: [
    {
      name: "sdfs",
      id: "34534"
    },
    {
      name: 'sdfs',
      id: "35344"
    }
  ]
}

Pacharapol Withayasakpunt • Feb 10 '20 • Edited

I have seen it stored like this (which is actually Anki)

'\x1f' + [ ...(string[]) ].join('\x1f') + '\x1f'

But if I had to do it more complex, like more fields, and rename-able, I would definitely do it the 2st way.

I wouldn't rely on JSON Object, if I can avoid.

Oscar • Feb 10 '20

Hmm, did you discard creating another table? What was the reason behind?

Yashu Mittal • Feb 10 '20

What do you mean by creating another table?

In the second option I am creating another table name tags to store the connection between the posts and tags.

Oscar • Feb 10 '20

I'm sorry, I totally missed. I just though that you would storage the array in a varchar (or any lob type) using JSON o separating it with |.

I would go for this three tables: posts, tags and post_tags (Or any naming you like).

posts --> All the posts
tags --> A master table with all the tags
post_tags --> A table with a post_id and tag_id to do the many to many relationship.

Yashu Mittal • Feb 10 '20

A table with a post_id and tag_id to do the many to many relationship.

You mean a single row of post_tags can contain multiple tags_id column.

Oscar • Feb 10 '20

I mean something like:

posts

post_id	other_value
1	something
2	something
3	something

tag_id	tag_text
1	tag1
2	tag2
3	tag3

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