SQL custom unit conversion

Question

I am looking for a solution for a custom unit conversion in SQL, the database my company used is Microsoft SQL server, I need to write a SQL to return a conversion factor based

Answer 1

I would use a conversion table and put in all combinations. So even if 5000g ->5kg -> 1 box, I would put gram -> box conversions as well. Something like this:

create table unit_unit_conv(
   from_unit varchar(10)   not null
  ,to_unit   varchar(10)   not null
  ,rate      decimal(10,6) not null
  ,primary key(from_unit, to_unit)
);

insert into unit_unit_conv values('kilogram', 'kilogram',   1);
insert into unit_unit_conv values('kilogram', 'gram',       1000);
insert into unit_unit_conv values('kilogram', 'box',        0.2);
insert into unit_unit_conv values('gram',     'gram',       1);
insert into unit_unit_conv values('gram',     'kilogram',   0.001);
insert into unit_unit_conv values('gram',     'box',        0.0002);
insert into unit_unit_conv values('box',      'box',        1);
insert into unit_unit_conv values('box',      'kilogram',   5);
insert into unit_unit_conv values('box',      'gram',       5000);

So whatever unit of measure you have, you can convert it into any unit by multiplying the quantity you have with the rate column in this table. So if you have a table of items like this:

create table items(
   item_id        varchar(10) not null
  ,item_qty       int not null
  ,item_qty_unit  varchar(10)
);

insert into items values('chicken', 5,    'kilogram');
insert into items values('babies',  5000, 'gram');
insert into items values('beef',    1,    'box');

...and you want to convert everything to boxes, you would query the data like this:

select i.item_id
      ,i.item_qty    as qty_original
      ,item_qty_unit as qty_unit_original
      ,i.item_qty * c.rate as box_qty
  from items          i
  join unit_unit_conv c on(i.item_qty_unit = c.from_unit)
 where c.to_unit = 'box';

+---------+--------------+-------------------+----------+
| item_id | qty_original | qty_unit_original | box_qty  |
+---------+--------------+-------------------+----------+
| chicken |            5 | kilogram          | 1.000000 |
| babies  |         5000 | gram              | 1.000000 |
| beef    |            1 | box               | 1.000000 |
+---------+--------------+-------------------+----------+