How to model inheritance of two tables MySQL

Question

I have some tables where I store data and depending on the type of person (worker / civil) that did a job I want to store it in an event table, now these guys rescue an animal (there is an animal table).

Finally, I want to have a table to store the event that a guy (worker / civil), saved an animal, but how should I add a foreign key or how to know the id value of the civil or worker that did the job?

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

How to reflect the design of the following diagram in MySQL?

enter image description here

Additional details

I have modelled it the following way:

DROP TABLE IF EXISTS `tbl_animal`; 
CREATE TABLE `tbl_animal` (
 id_animal INTEGER NOT NULL PRIMARY KEY AUTO_INCREMENT,
 name VARCHAR(25) NOT NULL DEFAULT "no name",
 specie VARCHAR(10) NOT NULL DEFAULT "Other",
 sex CHAR(1) NOT NULL DEFAULT "M",
 size VARCHAR(10) NOT NULL DEFAULT "Mini",
 edad VARCHAR(10) NOT NULL DEFAULT "Lact",
 pelo VARCHAR(5 ) NOT NULL DEFAULT "short",
 color VARCHAR(25) NOT NULL DEFAULT "not defined",
 ra VARCHAR(25) NOT NULL DEFAULT "not defined",
 CONSTRAINT `uc_Info_Animal` UNIQUE (`id_animal`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `tbl_animal` VALUES (1,'no name', 'dog', 'M','Mini','Lact','Long','black','Bobtail');
INSERT INTO `tbl_animal` VALUES (2,'peluchin', 'cat', 'M','Mini','Lact','Long','white','not defined');
INSERT INTO `tbl_animal` VALUES (3,'asechin', 'cat', 'M','Mini','Lact','Corto','orange','not defined');
DROP TABLE IF EXISTS `tbl_person`; 
CREATE TABLE `tbl_person` (
 type_person VARCHAR(50) NOT NULL primary key 
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `tbl_person` (type_person) VALUES ('Worker');
INSERT INTO `tbl_person` (type_person) VALUES ('Civil');
DROP TABLE IF EXISTS `tbl_worker`; 
CREATE TABLE `tbl_worker`(
 id_worker INTEGER NOT NULL PRIMARY KEY,
 type_person VARCHAR(50) NOT NULL , 
 name_worker VARCHAR(50) NOT NULL , 
 address_worker VARCHAR(40) NOT NULL DEFAULT "not defined", 
 delegation VARCHAR(40) NOT NULL DEFAULT "not defined",
 FOREIGN KEY (type_person) REFERENCES `tbl_person` (type_person),
 CONSTRAINT `uc_Info_worker` UNIQUE (`id_worker`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `tbl_worker` VALUES (1,'Worker','N_CEDENTE1', 'DIR Worker 1', 'DEL');
INSERT INTO `tbl_worker` VALUES (2,'Worker','N_worker1', 'DIR Worker 2', 'DEL');
INSERT INTO `tbl_worker` VALUES (3,'Worker','N_worker2', 'address worker','delegation worker'); 
DROP TABLE IF EXISTS `tbl_civil`; 
CREATE TABLE `tbl_civil`(
 id_civil INTEGER NOT NULL PRIMARY KEY,
 type_person VARCHAR(50) NOT NULL ,
 name_civil VARCHAR(50) ,
 procedence_civil VARCHAR(40) NOT NULL DEFAULT "Socorrism", 
 FOREIGN KEY (type_person) REFERENCES `tbl_person` (type_person),
 CONSTRAINT `uc_Info_civil` UNIQUE (`id_civil`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `tbl_civil` VALUES (1,'Civil','N_civil1' , 'Socorrism');
 
CREATE TABLE `tbl_event` (
 id_event INTEGER NOT NULL,
 id_animal INTEGER NOT NULL,
 type_person VARCHAR(50) NOT NULL , 
 date_reception DATE DEFAULT '2000-01-01 01:01:01',
 FOREIGN KEY (id_animal) REFERENCES `tbl_animal` (id_animal),
 FOREIGN KEY (type_person ) REFERENCES `tbl_person` (type_person ),
 CONSTRAINT `uc_Info_ficha_primer_ingreso` UNIQUE (`id_animal`,`id_event`) 
)ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `tbl_event` VALUES (1,1, 'Worker','2013-01-01 01:01:01' );
INSERT INTO `tbl_event` VALUES (2,2, 'Civil','2013-01-01 01:01:01' );

However, is there a way to get rid of nulls?

The queries I have are:

SELECT a.*,b.*,z.*
FROM tbl_event a
 left JOIN tbl_worker b
 ON a.type_person = b.type_person
 left JOIN tbl_animal z
 ON z.id_animal = a.id_animal ;
SELECT a.*,b.*,z.*
FROM tbl_event a
 left JOIN tbl_civil b
 ON a.type_person = b.type_person
 left JOIN tbl_animal z
 ON z.id_animal = a.id_animal ;

Here is an updated sqlfiddle .

Answer 1

Since I made the diagram, I better answer ;)

Current relational databases unfortunately don't support the inheritance directly, therefore you need to transform it into "plain" tables. There are generally 3 strategies for doing so:

1. All classes¹ in a single table with NULL-able non-common fields.
2. Concrete classes² in separate tables. Abstract classes don't have the tables of their own.
3. All classes in separate tables.

For more on what this actually means and some pros and cons, please see the links provided in my original post, but in a nutshell the (3) should probably be your default unless you have a specific reason for one of the other two. You can represent the (3) in the database simply like this:

CREATE TABLE person (
 person_id int PRIMARY KEY
 -- Other fields...
);
CREATE TABLE civil (
 civil_id int PRIMARY KEY REFERENCES person (person_id)
 -- Other fields...
);
CREATE TABLE worker (
 worker_id int PRIMARY KEY REFERENCES person (person_id)
 -- Other fields...
);
CREATE TABLE event (
 event_id int PRIMARY KEY,
 person_id int REFERENCES person (person_id)
 -- Other fields...
);

Unfortunately, this structure will let you have a person that is neither civil nor worker (i.e. you can instantiate the abstract class), and will also let you create a person that is both civil and worker. There are ways to enforce the former at the database level, and in a DBMS that supports deferred constraints³ even the latter can be enforced in-database, but this is one of the few cases where using the application-level integrity might actually be preferable.

---

¹ person, civil and worker in this case.

² civil and worker in this case (person is "abstract").

³ Which MySQL doesn't.

• Climax: You claim current relational databases don't support inheritance. What about postgresql? postgresql.org/docs/9.6/static/ddl-inherit.html
• Branko Dimitrijevic: @Climax I'm aware of PostgreSQL, but its implementation is only partial. From your link: "Other types of constraints (unique, primary key, and foreign key constraints) are not inherited."
• Baurzhan: @Branko Dimitrijevic, shouldn't we join all N children tables (2 in this case) for fetching person details (civil/worker) for particular event? When we do where childEntity.id = given_person_id, we don't know in advance, which child stores appripriate reference and need to look through all of them, right?
• Branko Dimitrijevic: @Baurzhan Yes, if you don't know in advance the type of the person you want to fetch, then you need to "probe" (via LEFT JOINs) all of the child tables (civil and worker). This is probably not a problem in this case, but may become a performance issue when there are many of them. in which case you may consider introducing a type discriminator into the parent table (person).

Answer 2

There is no need for distinct Civil_ID and Worker_ID; just continue to use Person-ID as the key for all three tables: Person, Civil, and Worker. Add a column PersonType to Person with the two values "Civil" and "Worker".

This now represents the two sub-classes CivilClass and WorkerClass of the abstract base class PersonClass as sub-entities Civil and Worker of the base entity Person. You get a nice correspondence between the data model in the DB with the object model in the application.

• edgarmtze: I have done a sqlfiddle sqlfiddle.com/#!2/1f6a4/1 but I do not know how to join other table, could you please point your answer here in the sqlfiddle?
• Branko Dimitrijevic: There are no "distinct" civil_id and worker_id - they are the same thing as person_id, just named differently - look at the FK1 (foreign key) marker in front of them.

Answer 3

Your case is an instance of class/subclass modeling. Or, as you have diagrammed it in ER, generalization/specialization.

There are three techniques that will help you design mysql tables to cover this case. They are called Single Table Inheritance, Class Table Inheritance, and Shared Primary key. You can read up on them in the info tab from the corresponding tag over in SO.

https://stackoverflow.com/tags/single-table-inheritance/info

https://stackoverflow.com/tags/class-table-inheritance/info

https://stackoverflow.com/tags/shared-primary-key/info

Single table inheritance is useful for simple cases where the presence of NULLs doesn't cause problems. Class table inheritance is better for more complicated cases. Shared primary key is a good way to enforce one-to-one relationships, and to speed up joins.

Answer 4

You can create a person type table and add a field to all tables needing the type enforcement. Then create foreign keys. Here is an example deriving from yours...

 CREATE TABLE person_type (
 person_type_id int PRIMARY KEY
 -- data: 1=civil, 2=worker
 -- Other fields (such as a label)...
 );
 CREATE TABLE person (
 person_id int PRIMARY KEY
 person_type_id int FOREIGN KEY REFERENCES person_type (person_type_id)
 -- Other fields...
 );
 CREATE TABLE civil (
 civil_id int PRIMARY KEY REFERENCES person (person_id)
 person_type_id int FOREIGN KEY REFERENCES person (person_type_id)
 -- Other fields...
 );
 CREATE TABLE worker (
 worker_id int PRIMARY KEY REFERENCES person (person_id)
 person_type_id int FOREIGN KEY REFERENCES person (person_type_id)
 -- Other fields...
 );
 CREATE TABLE event (
 event_id int PRIMARY KEY,
 person_id int REFERENCES person (person_id)
 -- Type is optional here, but you could enforce event for a particular type
 person_type_id int FOREIGN KEY REFERENCES person (person_type_id)
 -- Other fields...
 );