MySQL query to retrieve related data from another table - php

Question

I'm not too experienced with SQL queries (most times I used query builder from frameworks, like CodeIgniter and Laravel). Now, I need to retrieve data from a relational DB which have two tables: one for entity entries and other for complemental data of entities. For example, see below:
tbl_posts
id
name
slug
1
Lorem ipsum
lorem-ipsum
2
Testing post
testing-post
3
Hello world
hello-world
tbl_posts_meta
id
post_id
key
value
1
1
first_name
John
2
1
last_name
Doe
3
1
rating
5
4
2
parent
1
5
2
rating
3
6
3
rating
4
In this example, I need to retrieve an array of objects in this format:
[
{
id: 1,
name: "Lorem ipsum",
slug: "lorem-ipsum",
data: {
first_name: "John",
last_name: "Doe",
rating: 5,
}
},
{
id: 2,
name: "Testing post",
slug: "testing-post",
data: {
parent: 1,
rating: 3,
}
},
{
id: 3,
name: "Hello World",
slug: "hello-world",
data: {
rating: 4,
}
},
]
I've tried using subquery to handle this, but I'm reveiving Cardinality violation: 1241 Operand should contain 1 column(s) error. My query looks like this:
SELECT *,(SELECT * FROM tbl_posts_meta WHERE "post_id" = "tbl_posts"."id") as data FROM tbl_posts;
I've already tried using JOIN, but the result looks more away from expected (I have one "key" property and one "value" property in result, containing the last found entry in tbl_posts_meta).
SELECT * FROM tbl_posts INNER JOIN tbl_posts_meta ON tbl_posts_meta.post_id = tbl_posts.id;
Is there someway I can retrieve desired results with one query? I don't want to do this by applogic (like first retrieving data from tbl_posts and appending another query on "data" property, that returns all data from tbl_posts_meta), since this way may cause database overload.
Thanks!

Answer 1

The code is not shown right in the comment so here it is for you to check, sorry I can't contribute more to the theoretical debate:
select
tp.id,
tp.name,
tp.slug,
(select group_concat(`key`, "=",value separator ';' ) from tbl_posts_meta where post_id= tp.id )as datas
from tbl_posts tp

Answer 2

You can follow these simple steps to get to your needed json:
join the two tables to have centralized data
generate a tbl_posts_meta JSON-like string by aggregating on your columns of interest (tbl_posts.id, tbl_posts.name, tbl_posts.slug) using a GROUP_CONCAT aggregation function. Since your keys can either be integers or strings, you need to do a quick check using the CAST(<col> AS UNSIGNED), which will return 0 if the string is not a number.
use the JSON_OBJECT function to transform each row into a JSON
use the JSON_ARRAYAGG function to merge all jsons into a single json.
WITH cte AS(
SELECT p.id,
p.name,
p.slug,
CONCAT('{',
GROUP_CONCAT(
CONCAT('"',
m.key_,
'": ',
IF(CAST(m.value_ AS UNSIGNED)=0,
CONCAT('"', m.value_, '"'),
CAST(m.value_ AS UNSIGNED)) )
ORDER BY m.id ),
'}') AS data_
FROM tbl_posts p
INNER JOIN tbl_posts_meta m
ON p.id = m.post_id
GROUP BY p.id,
p.name,
p.slug
)
SELECT JSON_ARRAYAGG(
JSON_OBJECT("id", id,
"name", name,
"slug", slug,
"data", CAST(data_ AS JSON)))
FROM cte
Check the demo here.

Answer 3

After some research, testing and reading the queries you posted here, I've finally reached the result I needed. The final query is structured this way:
SELECT *, (
SELECT JSON_ARRAYAGG(
JSON_OBJECT(
key, value
)
) FROM tbl_posts_meta WHERE post_id = post.id
) AS data
FROM tbl_posts;
Translating it to Laravel's Eloquent query builder, and using PHP's serialize/unserialize functions, the end result looks like this:
$posts = Post::query()
->select("*")
->selectRaw('(SELECT JSON_ARRAYAGG(JSON_OBJECT(key, value)) FROM tbl_posts_meta WHERE post_id = posts.id) as data');
return $posts->get()->map(function($post){
$meta = [];
foreach($post->data as $dataItem){
foreach($dataItem as $key => $value){
$meta[$key] = unserialize($value);
}
}
$post->data = $meta;
return $post;
});
Thank you for your support and attention! :)