SQL query for key value table with 1:n relation - sql

Question

I have a table in which I want to store images. Each image has arbitrary properties that I want to store in a key-value table.
The table structure looks like this
id
fk_picture_id
key
value
1
1
camera
iphone
2
1
year
2001
3
1
country
Germany
4
2
camera
iphone
5
2
year
2020
6
2
country
United States
Now I want a query to find all pictures made by an iphone I could to something like this
select
fk_picture_id
from
my_table
where
key = 'camera'
and
value = 'iphone';
This works without any problems. But as soon as I want to add another key to my query I am get stucked. Lets say, I want all pictures made by an iPhone in the year 2020, I can not do something like
select
distinct(fk_picture_id)
from
my_table
where
(
key = 'camera'
and
value = 'iphone'
)
or
(
key = 'year'
and
value = '2020'
)
...because this selects the id 1, 4 and 5.
At the end I might have 20 - 30 different criteria to look for, so I don't think some sub-selects would work at the end.
I'm still in the design phase, which means I can still adjust the data model as well. But I can't think of any way to do this in a reasonable way - except to include the individual properties as columns in my main table.

Answer 1

A pattern you can consider here is to build a table of search parameters, then simply join this to your target table.
You would first create a temporary table with key and value columns then insert into it the search criteria values, any number of values you wish.
Using a CTE in place of a temporary table might look like:
with s as (
select 'camera' key, 'iphone' value
union all
select 'year', '2020'
)
select distinct t.fk_picture_id
from s
join t on t.key=s.key and t.value=s.value

Answer 2

The solution I found - thanks to this article
How to query data based on multiple 'tags' in SQL?
is that I made some changes to the database model
picture
id
name
1
Picture 1
2
Picture 2
And then I created a table for the tags
tag
id
tag
100
Germany
101
IPhone
102
United States
And the cross table
picture_tag
fk_picture_id
fk_tag_id
1
100
1
101
2
101
2
102
For a better understanding of the datasets
Picture
Tagname
Picture 1
Germany & Iphone
Picture 2
United States & IPhone
Now I can use the following statement
SELECT *
FROM picture
INNER JOIN (
SELECT fk_picture_id
FROM picture_tag
WHERE fk_tag_id IN (100, 101)
GROUP BY fk_picture_id
HAVING COUNT(fk_tag_id) = 2
) AS picture_tag
ON picture.id = picture_tag.fk_picture_id;
The only thing I need to do before the query is to collect the IDs of the tags I want to search for and put the number of tags in the having count statement.
If someone needs the example data, here are the sql statements for the tables and data
create table picture (
id integer,
name char(100)
);
create table tag (
id integer,
tag char(100)
);
create table picture_tag (
fk_picture_id integer,
fk_tag_id integer
);
insert into picture values (1, 'Picture 1');
insert into picture values (2, 'Picture 2');
insert into tag values (100, 'Germay');
insert into tag values (101, 'iphone');
insert into tag values (102, 'United States');
insert into picture_tag values (1, 100);
insert into picture_tag values (1, 101);
insert into picture_tag values (2, 101);
insert into picture_tag values (2, 102);