Return multiple columns from subquery as Columns - sql

Question

Here is my data.
Products: <Where all list of SKU are stored>
Prod_ID | Prod_Desc | Base_Unit_ID
1 | Custom Product | 1
UOM: <Masterlist for Unit of measures>
UOM_ID | Desc
1 | Piece
2 | Box
3 | Case
UOM_Conversion: <From base unit. Multiplier is how many base unit in a To_Unit>
Prod_ID | from_Unit_ID | Multiplier | To_Unit_ID
1 | 1 | 100 | 2
1 | 1 | 400 | 3
Given This Data. How Can I display it like this?
Product | Base Unit | Pack Unit | Multiplier | Case Unit | Multiplier
Custom Product | Piece | Box | 100 | Case | 400
I have tried Left Join Lateral but sadly. What it does is return two rows.
The reason I want to do this is because Im developing a module which stores products with multiple Unit of measure(Max of 3). So I dont want to create 3 columns of Unit, Pack and Case hence the reason I created the UOM_conversion table.

Answer 1

I think there is a global misconception here but I will give you it after the solution for your problem. I advise you to look at it.
So firstly : what you need here is the crosstab() function.
CREATE EXTENSION tablefunc;
For the following script :
create table uom
(
uom_id int primary key
, description varchar (250)
);
create table products
(
prod_id int primary key
, prod_desc varchar(250)
, base_unit_id int references uom (uom_id)
);
create table uom_conversion
(
prod_id int references products (prod_id)
, from_unit_id int references uom (uom_id)
, multiplier int
, to_unit_id int references uom (uom_id)
);
insert into uom values (1, 'Piece'), (2, 'Box'), (3, 'Case');
insert into products values (1, 'Custom Product', 1);
insert into uom_conversion values (1,1,100,2), (1,1,400,3);
The request is :
select
p.prod_desc as "Product"
, u.description as "Base Unit"
, u2.description as "Pack Unit"
, final_res."1" as "Multiplier"
, u3.description as "Case Unit"
, final_res."2" as "Multiplier"
from crosstab(
'select
p.prod_id
, base_unit_id
, multiplier
from products p
inner join uom_conversion uc
on uc.prod_id = p.prod_id')
as final_res (prod_id int, "1" int, "2" int)
inner join crosstab('select
uc.prod_id
, u.description
, uc.to_unit_id
from uom_conversion uc
inner join uom u
on u.uom_id = uc.to_unit_id')
as final_res_2 (prod_id int, "Box" int, "Case" int)
on final_res.prod_id = final_res_2.prod_id
inner join products p
on p.prod_id = final_res.prod_id
inner join uom u
on p.base_unit_id = u.uom_id
inner join uom u2
on u2.uom_id = final_res_2."Box"
inner join uom u3
on u3.uom_id = final_res_2."Case";
This is solving your problem. BUT : How do you know the order of what is pack_unit and what is the case_unit? I think from this question a lot more will come up.