Mysql如何创建EQUAL约束？

Question

Is there a way to create an "EQUAL" constraint in Mysql?

Imagine I have a table like this

tb_1:

id | id_fk | order | id_tb_2 | Column D
----------------------------------------
1  | 1     | 1     | 10      | AAA
2  | 1     | 2     | 20      | BBB
3  | 2     | 1     | 30      | CCC
4  | 3     | 1     | 40      | DDD

And:

tb_2:

id_tb_2 | id_type
------------------
10      | 100
20      | 200
30      | 300
40      | 400

And:

tb_3:

id_type | model    | other_prop
--------------------------------
100     | model_A  | A
200     | model_A  | B
300     | model_B  | C
400     | model_C  | D

I want to make sure that tb_2.id_type.model are going to be equal for same values of id_fk and different order .

So I have a UNIQUE constraint for [id_fk, order]

And I need a EQUAL constraint for [id_fk, id_tb_2.id_type.model]

This way, I shouldn't be able to create a value in tb_1 like:

id | id_fk | order | id_tb_2 | Column D
----------------------------------------
1  | 1     | 1     | 10      | AAA
2  | 1     | 2     | 20      | BBB
5  | 1     | 3     | 30      | EEE  <--- This has a different model in tb_2

How can I create one?

Answer 1

A solution using proper relational normalization is to drop the tb_1.Column D column.

Then the reference to tb_2.id_tb_2 and by extension to tb_3.id_type will be unambiguous.

tb_1:

id | id_fk | order | id_tb_2
----------------------------
1  | 1     | 1     | 10
2  | 1     | 2     | 20
3  | 2     | 1     | 30
4  | 3     | 1     | 40

In other words, by including Column D in tb_1, you're violating Third Normal Form , because Column D is an attribute that depends on something other than the primary key of tb_1.

But I'm not sure you've told us the full scope of the problem you're trying to solve.

Answer 2

You can't.

The only constraints that relate data from multiple tables are FOREIGN KEY constraints; and they are very limited about the relationship they check: a list of columns against another list of columns on another/same table.

All other relational constraints (such as UNIQUE or CHECK ) have a table scope.