MySQL Many-to-One relationship

Question

I'm having a hard time grasping this one.

I have a table with hosts , each with a dozen or so rows, and a table with points , which is actually a table with timeline information about a host.

A host has many points, which can be translated to a timeline on that host's conditions over time.

How can I translate that into an SQL query that groups x points with their respective host ?

Example:

Host: 
    id:1
    address: 123.456.7.8
    name: FooBar
    pickles: The good kind
    timeline:
        Point:
            host_id:1
            timestamp: 123456
            parameter: 123
        Point:
            host_id:1
            timestamp: 123456
            paramter:456

Important: I also want to limit the number of entries in this timeline column.

I've been grinding my gears about this one for quite some time now, and I'd appreciate some help.

Answer 1

您可以执行类似的子查询

SELECT h.*, (SELECT COUNT(*) FROM points WHERE host = h.host) as points FROM hosts h

Answer 2

Looking at your example something like

    select id, address, name, pickles,timestamp,parameter from host
 left join timeline on host.id = timeline.host_id 
order by host.id

However, if a host can have multiple occurances, I would want to see an example where you as a human had worked out from example data how to match them

Answer 3

select * from hosts right join timeline where host.id = timeline.host_id

Will return all points, with host information on the same row.

You can use the GROUP BY clause and COUNT functions to aggregate by the host_id and get the number of points:

select count(host_id), hosts.id from hosts right join timeline where host.id = timeline.host_id group by host.id