创建两个具有相同主键的子类型（或子）表，该主键也是父表的外键 sql

Question

I have the following tables:

CREATE TABLE employee(
    id INT(11) UNSIGNED PRIMARY KEY,
    namee VARCHAR(60) NOT NULL)
ENGINE = InnoDB;

CREATE TABLE consierge(
    id INT(11) UNSIGNED PRIMARY KEY,
    namee VARCHAR(60) NOT NULL, 
    zonee VARCHAR(20), 
    functionn VARCHAR(20),
    FOREIGN KEY (id) REFERENCES employee(id))
ENGINE = InnoDB;

CREATE TABLE guard(
    id INT(11) UNSIGNED PRIMARY KEY,
    namee VARCHAR(60) NOT NULL, 
    rol VARCHAR(20) NOT NULL,
    wtime ENUM ('Day','Night'),
    FOREIGN KEY (id) REFERENCES employee(id)) 
ENGINE = InnoDB;

When I try to create an consierge or guard, it firts verify the existence of that id in employee, which is fine. The problem is that, ie, when an id is already in guard (hence in employee) and I create an consierge with that same id, the consierge gets created, so, an employee can be guard and consierge; and I want that an employee can be guard OR consierge EXCLUSIVElY.

Answer 1

You can do this -- without triggers -- using some additional logic with generated columns:

CREATE TABLE employee(
    id INT(11) UNSIGNED PRIMARY KEY,
    namee VARCHAR(60) NOT NULL,
    type varchar(255),
    c_id int generated always as (case when type = 'c' then id end) stored,
    g_id int generated always as (case when type = 'g' then id end) stored,
    check (type in ('c', 'g'))
);

CREATE TABLE consierge (
    id INT(11) UNSIGNED PRIMARY KEY,
    namee VARCHAR(60) NOT NULL, 
    zonee VARCHAR(20), 
    functionn VARCHAR(20),
    type varchar(255) generated always as ('c') stored,
    unique (type, id),
    FOREIGN KEY (id) REFERENCES employee(id)
);

CREATE TABLE guard(
    id INT(11) UNSIGNED PRIMARY KEY,
    namee VARCHAR(60) NOT NULL, 
    rol VARCHAR(20) NOT NULL,
    wtime ENUM ('Day','Night'),
    type varchar(255) generated always as ('g') stored,
    unique (stored, id),
    FOREIGN KEY (id) REFERENCES employee(id)
);

ALTER TABLE employee ADD CONSTRAINT fk_employee_c_id
    FOREIGN KEY (c_id) REFERENCES concierge(id);

ALTER TABLE employee ADD CONSTRAINT fk_employee_g_id
    FOREIGN KEY (g_id) REFERENCES guard(id);

This adds computed columns to allow the desired foreign key relationships.