外键来自SQL中一列中的两个表

Question

I have a order table and each order in that table belongs to either a device or part (these two have no specific intersection in fields).
So I need to store the did or pid as FK in order table.

"device"
+----+....
| id |<---------+
+----+....      :
: : : : .       :
                :
                :
"part"          :
+----+....      :
| id |<-------+ :
+----+....    : :
: : : : .     : :
              : :
              : :
     "order"  @ @
     +-----+-------+....
     | id  |  for  |....
     +-----+-------+....
     : : : : : : : : .

Now How should I do this?

• Add a type field to order table and store both pid or did on one column(eg for )
• Create an intermediate view with type , pid , did columns
• Create a higher level table (eg goods ) and make its PK to be FK on order , part , device

Which one is the best method? or some other method?

Answer 1

Either use exclusive foreign keys or inheritance , as explained here . Just be careful about CHECK, which is not enforced by MySQL, so you'll have to "emulate" it via trigger.

Please don't do the "type" approach, it can lead to problems .

Answer 2

I would go with two tables, one for the order and the other for item. Item table will have ID, type (part or device; P or D) and the rest of the details. In the order table, I will just have a FK to the Item Table ID.

Answer 3

If it was me, I would create both the following FK fields: did and pid . You would always have a value for one and not the other.

Your query would look something like the following:

SELECT o.*, COALESCE(p.descr, d.descr) AS Description,
            COALESCE(p.number, d.number) AS PNumber
FROM order o
LEFT JOIN device d ON o.did = d.id
LEFT JOIN part p ON o.pid = p.id