Question

Example:

Table: Unit
ID NAME  VALUE
1  Kilo  1000
2  Mega  1000000
3  Giga  1000000000

Table: Storage
ID Title    Drive_value Drive_unit_ID Cache_value Cache_unit_ID 
1  Seagate  100         3             400         1 
2  Scansoft 250         3             80          2 

Relations:

Storage.Cache_unit_ID references Unit.ID
Storage.Drive_unit_ID references Unit.ID 

Desired Goal:
To do a select * from Storage, display new columns ‘AS’ Drive and ‘AS’ Cache and display this by multiplying appropriate Unit.VALUE

 Title     Drive          Cache  
 Seagate   100000000000   400000          
 Scansoft  250000000000   80000000000          

Answer 1

You can join the same table any number of times as long as you alias the joined tables:

SELECT s.Title, u1.VALUE * s.Drive_value AS Drive, u2.VALUE * s.Cache_value AS Cache
FROM Storage s
INNER JOIN Unit u1 ON u1.ID = s.Drive_unit_ID
INNER JOIN Unit u2 ON u2.ID = s.Cache_unit_ID