mysql - 在 MySQL 中，如何对单个选择使用两次表引用？

Question

示例:

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 

关系:

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

期望目标: 要从存储中选择 *，请显示新列“AS”驱动器和“AS”缓存，并通过乘以适当的 Unit.VALUE 来显示此内容

 Title     Drive          Cache  
 Seagate   100000000000   400000          
 Scansoft  250000000000   80000000000          

Answer 1

只要为连接的表指定别名，您就可以多次连接同一个表:

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