Database design: how to design this peculiar relationship

Question

I'm pretty new to designing databases, and i'm encountering a challenge;

In a lab, DNA samples are prepared from tissue samples. These tissue samples have attributes such as species and location. It is possible, but not always the case, that different tissue samples are mixed before DNA sample preparation.

So, there are 3 tables, with following attributes:

A table, Tissue_sample :

- ID 
- location 
- species 

A table, Mixed_tissue_sample :

- ID 
- tissue samples which are mixed together 

A table, DNA_sample :

- ID 
- tissue_sample OR mixed_tissue_sample 

It should be possible to retrieve all tissue samples from which a DNA sample is prepared.

What is the most elegant way to fit this into a database? And is possible to do this in the MySQL workbench?

Answer 1

Define two fields in DNA table to reference a single or a mixed tissue sample. One of these fields will remain NULL.

tissue: id, location, species

mixed: id, ...

mixed_tissues: mixed_id, tissue_id

DNA: id, tissue_id, mixed_id

And yes, you can create these tables in MySQL workbench.

Answer 2

First, resolve the relation between Tissue_sample and Mixed_tissue_sample . 1 or more Tissue_sample defines a Mixed_tissue_sample, therefore the relation is 1:n and you need to have another column at Tissue_sample , which will serve as a foreign key to Mixed_tissue_sample . You can get the tissue samples of a mixed tissue sample by joining the two tables upon the identified of Mixed_tissue_sample .

Second, you need to resolve the DNA_sample . You need a foreign key to Tissue_sample and to Mixed_tissue_sample as well.

You can make sure a DNA_sample has no connection to both by creating a trigger which will prevent any inserts or updates which would result in a record of DNA_sample which has values for both foreign keys if you need that.

Answer 3

Use table inheritance.

Either put tissue sample and mixed tissue sample all in the same table (single table inheritance), with a "type" column,

or put them in two different tables with their primary key pointing to a common table (class table inheritance). DNA sample table would point at this common table.