WHERE statement with dynamic input - mysql

Question

I have two tables. The first one (item) is listing apartments. The second (feature) is a list of features that an apartment could have. Currently we list about 25 different features.
As every apartment can have a different set of features, I think it makes sense to have a 1:1 relationship between items and features table.
If in feature table for one the features the value is '1', this means that the linked apartment has this feature.
+-------------+------------+--------------+-------------+------------+
| table: item | | | | |
+-------------+------------+--------------+-------------+------------+
| id | created_by | titel | description | address |
+-------------+------------+--------------+-------------+------------+
| 10 | user.id | Nice Flat | text | address.id |
+-------------+------------+--------------+-------------+------------+
| 20 | user.id | Another Flat | text | address.id |
+-------------+------------+--------------+-------------+------------+
| 30 | user.id | Bungalow | text | address.id |
+-------------+------------+--------------+-------------+------------+
| 40 | user.id | Apartment | text | address.id |
+-------------+------------+--------------+-------------+------------+
+----------------+---------+--------------+----------------+--------------+------+
| table: feature | | | | | |
+----------------+---------+--------------+----------------+--------------+------+
| id | item_id | key_provided | security_alarm | water_supply | lift |
+----------------+---------+--------------+----------------+--------------+------+
| 1 | 10 | 1 | 0 | 0 | 1 |
+----------------+---------+--------------+----------------+--------------+------+
| 2 | 20 | 0 | 1 | 1 | 0 |
+----------------+---------+--------------+----------------+--------------+------+
| 3 | 30 | 1 | 1 | 0 | 1 |
+----------------+---------+--------------+----------------+--------------+------+
| 4 | 40 | 1 | 1 | 1 | 1 |
+----------------+---------+--------------+----------------+--------------+------+
I want to build a filter functionality so user can select to show only apartments with certain features.
e.g.:
$key_provided = 1;
$security_alarm = 1;
$water_supply = 0;
Does this database approach sounds reasonable for you?
What’s the best way to build a MySQL query to retrieve only apartments where the filter criteria match, keeping in mind that the number of features can be grow in future?

Answer 1

A better approach is to have a features table. In your case, they all seem to be binary -- yes or no -- so you can get away with:
create table item_features (
item_feature_id int auto_increment primary key,
item_id int not null,
feature varchar(255)
foreign key item_id references items(item_id)
);
The data would then have the positive features, so the first item would be:
insert into item_features (item_id, feature)
values (1, 'key_provided'), (1, 'lift');
This makes it easy to manage the features, particularly adding new ones. You might want to use a trigger, check constraint, or reference table to validate the feature names themselves, but I don't want to stray too far from your question.
Then checking for features is a little more complicated, but not that much more so. One method is explicitly using exists and not exists for each desired/undesired one:
select i.*
from items i
where exists (select 1
from item_features itf
where itf.item_id = i.item_id and
itf.feature = 'key_provided'
) and
exists (select 1
from item_features itf
where itf.item_id = i.item_id and
itf.feature = 'security_alarm'
) and
not exists (select 1
from item_features itf
where itf.item_id = i.item_id and
itf.feature = 'water supply'
);

Answer 2

For your existing data structure, you can filter as follows:
select i.*
from item i
inner join feature f
on f.item_id = i.id
and f.key_provided = 1
and f.security_alarm = 1
and f.water_supply = 0
This will give you all the apartments that satisfy the given criteria. For more criterias, you can just add more conditions to the on part of the join.
As a general comment about your design:
since you are creating a 1-1 relationship between apartments and features, you might as well consider having a single table to store them (spreading the information over two tables does not have any obvious advantages)
your design is OK as long as features do not change too often, since, basically, everytime a new feature is created, you need to add more columns to your table. If features are added (or removed) frequently, this can become heavy to manage; in that case, you could consider having a separated table where each (item, feature) tuple is stored in a different row, which will make this of things easier to do (with the downside that queries will get more complicated to write)