sql中创建主键的问题

Question

I have table that has null type of data and it need to be null

replacement parts 0,m-> Repair

So replacement parts can be part of repair but every repair don't need to have replacement parts and when I am creating tables I putted for replacement parts id null, but problem is when I am creating primary key it can be only with not null. What to do?

Answer 1

The question describes only one half of the relation between repairs and replacement parts. You stated that a repair may consist of zero ore more replacement parts.

But (1) can the same replacement part be taken in zero or more repairs, or (2) is a replacement part consumed by a repair such that it can occur only zero or one time in a repair?

It is necessary to make this clear, because (1) and (2) lead to different database schemes:

(1) is a many - to - many relationship and leads to a separate table connecting parts and repairs:

create table part (
    partId int primary key
);

create table repair (
    repairId int primary key
    );

create table part_repair (
    partId int references part (partId),
    repairId int references repair (repairId)
    primary key (partId, repairId)
    );

(2) part may be used in zero or one repair; store repair-id in part:

create table repair (
    repairId int primary key
    );

create table part (
    partId int primary key,
    repairId int references repair(repairId)
);

Note that repairId in part may take on NULL values for those parts that are not assigned to a repair.

Hope that helps a bit.