How to join a polymorphic table with its child tables? - sql

Question

I am sorry about not being able to articulate the title of the question or the description of this question better. However, I will give the schema, sample data and expected result. Please help me write a query for such a use case.
Schema of restaurants
id
name
item_type
item_id
Schema of foods
id
name
Schema of food_items
id
name
food_id
Sample data in restaurants
|---------------------|------------------|---------------------|------------------|
| id | name | item_type | item_id |
|---------------------|------------------|---------------------|------------------|
| 1 | Apple Crushers | food_items | 1 |
|---------------------|------------------|---------------------|------------------|
| 2 | Retro Cafe | foods | 2 |
|---------------------|------------------|---------------------|------------------|
| 3 | Fruit Mania | foods | 1 |
|---------------------|------------------|---------------------|------------------|
| 4 | Meat and Eat | NULL | NULL |
|---------------------|------------------|---------------------|------------------|
Sample data in foods:
|---------------------|------------------|
| id | Name |
|---------------------|------------------|
| 1 | Fruits |
|---------------------|------------------|
| 2 | Chocolates |
|---------------------|------------------|
Sample data in food_items
|---------------------|------------------|---------------------|
| id | Name | food_id |
|---------------------|------------------|---------------------|
| 1 | Apple | 1 |
|---------------------|------------------|---------------------|
| 2 | Mango | 1 |
|---------------------|------------------|---------------------|
I need to write a query such that I get this as my result.
|---------------------|------------------|---------------------|------------------|---------------------|------------------|
| r_id | r_name | food_id | food_name | food_item_id | food_item_name |
|---------------------|------------------|---------------------|------------------|---------------------|------------------|
| 1 | Apple Crushers | 1 | Fruit | 1 | Apple |
|---------------------|------------------|---------------------|------------------|---------------------|------------------|
| 2 | Retro Cafe | 2 | Chocolates | NULL | NULL |
|---------------------|------------------|---------------------|------------------|---------------------|------------------|
| 3 | Fruit Mania | 1 | Fruit | NULL | NULL |
|---------------------|------------------|---------------------|------------------|---------------------|------------------|
| 4 | Meat and Eat | NULL | NULL | NULL | NULL |
|---------------------|------------------|---------------------|------------------|---------------------|------------------|
p.s: It will also be very helpful if someone could come up with an appropriate title and description for this problem. I am lost for words to describe this.

Answer 1

You must join the food table twice and use COALESCE:
select
r.id,
r.name,
coalesce(f.id, fif.id) as food_id,
coalesce(f.name, fif.name) as food_name,
fi.id as food_item_id,
fi.name as food_item_name
from restaurants r
left join foods f on f.id = r.item_id and r.item_type = 'foods'
left join food_items fi on fi.id = r.item_id and r.item_type = 'food_items'
left join foods fif on fif.id = fi.food_id
order by r.id;

Answer 2

select r.id, r.name, foods.id, foods.name, food_items.id, food_items.name from restaurents as r
left join food_items on r.item_type = 'food_items' and r.item_id = food_items.id
left join foods on (r.item_type = 'foods' and r.item_id = foods.id) or (r.item_type = 'foods' and r.item_id = food_items.food_id)
This might be having some syntax issues related to table names, but it should work.

Answer 3

For your database, I would suggest another data model based on compound keys. This would guarantee data consistency and makes queries a tad simpler:
food_group
(
food_group_no integer not null,
name varchar(100) not null,
primary key (food_group_no)
);
food
(
food_group_no integer not null,
food_no integer not null,
name varchar(100) not null,
primary key (food_group_no, food_no)
);
restaurant
(
restaurant_no integer not null,
name varchar(100) not null,
food_group_no integer not null,
food_no integer null,
primary key (restaurant_id),
foreign key (food_group_no) references food_group(food_group_no),
foreign key (food_group_no, food_no) references food(food_group_no, food_no)
);
The query:
select
r.restaurant_no,
r.name as restaurant_name,
fg.food_group_no,
fg.name as food_group_name,
f.food_id,
f.name as food_name
from restaurants r
join food_group fg on fg.id = r.food_group_no
left join food f on f.food_group_no = r.food_group_no and f.food_no = r.food_no
order by r.id;