How to relate 3 tables depending on event

Question

I have a table that have information about different types of events that can be done by persons in two categories civil and worker

so for each one of them I have their respective tables

civil{  civil_id, name, age,telephone...} the  primary key is civil_id

worker{ worker_id, name, duty, department...} the  primary key is worker_id

then the event table has a list of all possible events

event {type_of_event} the  primary key is type_of_event

then I am planing to store information in other table

with eventype, the person that did the job (worker or civil)

id  event_type       date      person
-----------------------------------
1   type1         12-12-12     x 
2   type1         05-12-10     y
3   type2         02-12-12     y

Now in this design I do not know how to relate whose person did the job if, I would had only a kind of person (aka civil) i would only store the civil_id in person field in this last table....but how to know if it was civil or worker, do I need other intermediate table?

Answer 1

In this can, you cannot set up a foreign key because you have multiple parent. In order to do fast search or to avoid full table scan define an index on column person on table event and join the table using LEFT JOIN. eg,

SELECT  ....,
        COALESCE(b.name, c.name) AS personname
FROM    event a
        LEFT JOIN civil b
            ON a.person = b.civil_id
        LEFT JOIN worker c
            ON a.person = c.worker_ID

Adding INDEX

ALTER TABLE event ADD INDEX (person)

Answer 2

Generally, there are to ways to model this type of situation...

Using Exclusive Foreign Keys

In the event, both civil_id and worker_id are NULL-able, but there is also a constraint ensuring exactly one of them is non-NULL at any given time:

CHECK (
    (civil_id IS NOT NULL AND worker_id IS NULL)
    OR (civil_id IS NULL AND worker_id IS NOT NULL)
)

Using Inheritance¹

For more on inheritance, take a look at "Subtype Relationships" chapter in the ERwin Methods Guide and at this post.

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¹ Aka. category, subtyping, subclassing, generalization hierarchy...