我应该在表格中创建一个新字段，还是只选择第二个表格的MAX

Question

On this question I received an answer that worked well. I am now wondering if there is a possible better structure.

I have two tables.

Projects : id, title
Status : project_id, status_id, created(DATETIME)

At the moment to get the status of my project, I get the project ID and pull the latest row out of the status table based on the project id. To get this latest row is quite a hassle.

Should I rather change the schema to this?

Projects : id, title, current_status_id(FK)
Status : id(PK), project_id, status_id, created(DATETIME)

Then I can just join the tables with the FK and get the row that I want without looking for the latest?

EDIT :

So I want something like this

SELECT * FROM projects
LEFT JOIN status on projects.id = status.project_id
WHERE projects.id = 1

But I want only the latest record in the status table.

EDIT 2 :

So I want something like this

SELECT * FROM projects
LEFT JOIN status on projects.id = status.project_id

But for each project returned, only get the latest status record for that project_id from status.

Answer 1

How is this a hassle?

SELECT project_id, status_id, created 
FROM Status 
WHERE project_id = the-id
ORDER BY created DESC
LIMIT 1;

Or, if you need a list of multiple projects:

SELECT a.project_id, a.status_id, a.created 
FROM Status a
LEFT JOIN Status b
ON a.project_id = b.project_id
AND b.created > a.created
WHERE a.project_id IN(id1, id2, id3) AND b.project_id IS NULL;

So, with project data:

SELECT Projects.*, Status.*
FROM Projects
LEFT JOIN Status 
ON Status.project_id = Projects.id
WHERE Projects.id = the-id
ORDER BY Status.created DESC
LIMIT 1;

Or:

SELECT Projects.*, Status.*
FROM Projects
LEFT JOIN Status a
ON a.project_id = Projects.id
LEFT JOIN Status b
ON a.project_id = b.project_id
AND b.created > a.created
WHERE b.project_id IS NULL;

Answer 2

That's one way to do it.

You might not even need the current_status field to be an FK; why not just store the value? You could get into odd circular references otherwise.

Another way would be to store a status archive, separate of the projects table. Every time the status changes, insert the current status into the archive table, and change the projects.status value.