One to one relationship involving multiple tables

问题

Say,

I have 3 tables.

User which contains basic info about the users.

SectionA which contains more info about the user.

SectionB which also contains more info about the user.

There can only be one SectionA and SectionB data for each user.

My idea was to create a table design like this:

id  name   section_a_id   section_b_id
1   matt   1               1

Problem is, section_a_id and section_b_id cannot be auto incremented since they are not primary keys.

So I tried a different approach and decided that the id primary key in User should be a foreign key that refers to section_a_id and section_b_id`. But I'm unable to do so since mysql will only allow a reference to one table.

So how should I approach this situation?

回答1:

If it's one to one relation, it will always be easier to combine the three tables into one big table, with nullable columns for Section tables.

Some positive points I can see for this approach:

• Easier insert, update and delete operations.
• Faster data retrieval when there are less joins to use.
• Less indexes space, because you are indexing the primary key for one table instead of three tables.

回答2:

It seems you have:

• 1-n User-SectionA relation
• 1-n User-SectionB relation

so the foreign keys have to be in SectionA and SectionB tables and they have to be the User table primary key.

回答3:

There can only be one SectionA and SectionB data for each user.

This begs the question of why you are storing the data in different tables. That is sometimes a fine thing to do -- different users might be in different tables.

One solution is to have the primary key of SectionA and SectionB be a foreign reference to the UserId. Yes, this is allowed.

Another solution is to have the sectionA_id and sectionB_id in the users table. Then you can have an auto-incremented id in each of the referenced tables.

回答4:

Rare uses for 1:1

• You really have too many columns;
• The table is huge and you don't want to ALTER the table to add more;
• One of the tables is mostly optional, and you use LEFT JOIN to get nulls.

Righting some AUTO_INCREMENT misconceptions

• The column does not have to be the PRIMARY KEY, nor even UNIQUE; it only needs to be the first column in some index.
• You do not have to have an AUTO_INCREMENT column at all. It is often better to use a "natural" PK -- some column (or combination of columns) that is inherently UNIQUE.

When building a pair of 1:1 tables, this is one way to do it

1. Build the first table with a PRIMARY KEY that is AUTO_INCREMENT.
2. Build the second table with a PRIMARY KEY that is identical to the other, but not AUTO_INCREMENT.
3. When inserting a row, use LAST_INSERT_ID() to get the id.
4. Use that value for explicitly specifying the id for the second table.

来源：https://stackoverflow.com/questions/45914666/one-to-one-relationship-involving-multiple-tables